This question is about big trees that can contain an entire city.
So generally speaking: What limits a plant's size, anything from a tree to a flowering plant?
What environment needs to exist for a daisy to become the size of a dinner table (for example)?
For the purpose of this question you should assume a reasonably conventional environment similar to that found here on Earth. For discussion of low and zero gravity plants you can visit this question.
Anything is possible.
The major factor in the size of a plant is how far and can get water and food to the various parts of the plants. Water is more important because food can be produced by leaves almost anywhere in the plant. Massive trees like Redwoods get the extra water they need by absorbing it through their leaves, instead of bringing it all the way up from the roots. This makes massive non-vascular plants near impossible.
Plants have a certain lifespan. Something like a daisy doesn't grow as big as a dinner plate because it dies. Its DNA declares that it dies after it has produced its seeds. A Redwood, on the other hand, can live many, many years, so they can grow very big. The plant needs to be able to live a long lifespan, without getting eaten, to grow big. While it is possible that plant could grow very fast, it would need very good conditions (practically infinte amounts of food and water), and the need to do it. Generally larger plant doesn't die as easily, and can more easily produce seeds, the ultimate goal of most plants.
A plant needs to have the proper structure for its size. A large plant needs to be vascular, with roots, stems and leaves. This allows it to transport large amounts water and food to the necessary places. The plant also needs to have the correct size of stem and branches. A Redwood has a massize trunk (stem). Redwoods don't stand up on the stems of daisies.
To better illustrate my points, I'll explain how the example could be done. A daisy, with its exact genetic could and build, could not grow to the size of a dinner plate. While technically it could transport the water and food needed, it would not live long enough to grow that big, nor would it be able to support its size.
Flowers as big as dinner plates do exist, but they generally sit on the ground. For a daisy-like plant to have that big a flower, it would need a massive, probably wood like, stem. It would likely take a long time to grow. Which breaks the lifespan rule. For it to make sense for a daisy to grow this long, the plant would have to be perennial, meaning it would die over the winter. This plant would be closer to a tree than a daisy. After it reached a certain size, it might produce large, daisy-like flowers. It would probably produce 1 flower per year, and release hundreds of seeds every year. But you probably wouldn't call the plant a daisy.
It's conceivable that a plant that is able to acquire nutrients and moisture as it grows (rather than from a central root) would be able to just keep growing horizontally until it runs out of space or dies. Vertically it would be constrained by its ability to support itself and to draw water.
A daisy in particular doesn't grow to the size of a dinner plate because the complex set of biochemical processes that cause a daisy to grow the way it does and to become a daisy rather than say a water buffalo result in it growing to be daisy sized and then stopping.
A simple scaling of a daisy would cause the same square-cube problems as scaling pretty much anything. Doubling the linear dimensions of a daisy would make it 4 times stronger, but 8 times heavier. A dinner plate sized daisy would collapse under its own weight.
We could selectively breed daisies to get bigger, but as they did, we'd need to breed them to support their new size. A daisy bred to be dinner plate sized would need to have a radically different structure from a a scaled up daisy (Stem would need to get thicker faster than the other scaling, a tougher woodier composition, restructuring the head to be lighter for its size, etc.)
DonyorM has made some good points, but I feel that I should add that the limiting factors in the height of trees (the tallest plants known to us) are many:
Trees grow tall in response to competition for light. The tallest trees occur in forests, not out on the plains, even though the two may be the same species or even genetically identical individuals. So, a tree need not be much taller than the tallest of its neighbors.
In order to grow, a tree must acquire the required energy. Leaves can only cram in so much chlorophyll into a given volume (which must be thin to maximize light absorption), and chlorophyll can only catch a fraction of the available energy and use it to photosynthesize Carbon dioxide and water to glucose, which the tree can use as an energy source. As a tree grows larger, it must provide energy to more living but non-photosynthesizing wood, and there reaches a point where the energy available is all going to keeping the tree alive, and there is none to spare for further growth.
In trees, water makes its way from the roots to the leaves by transpiration, the water flowing up narrow vessels. The combination of transpiration, root pressure and capillary action in the vessels can account for water columns of up to 1.5km in height in terrestrial conditions. However, the higher the water column, the more likely it will break (particularly when there is a lack of groundwater), rendering the vessel useless for transporting more water.
The taller the tree becomes, the wider its base must become in order to support its own weight. This again depends on gravity and the density and compressive strength of the wood.
So, given sufficient competition for light, if a more efficient photosynthetic pigment or combination of pigments was used, and the wood was strong and light, there would be no reason why a tree couldn't reach a height on the order of several hundred to a kilometer in height, perhaps more. Such trees would have very thick bases and may well live in very wet areas in order to provide the amount of water that would be required.
Beyond enviromental reasons. There are very important genetic reasons, It has been proven in a few species in special a poblar tree that the PXY kinase receptor and its peptide ligand CLE 41 regulate the cambial cell division which is seen as growth in heigh and volume yearly. Altering the expression on this two elements does modify how much a tree grows and how much it increases volume every year as per its genetics. Altering this two elements in a tree can double the rates of growth for a tree in same environemental conditions. Please see reference "http://www.cell.com/current-biology/abstract/S0960-9822%2815%2900162-1"
Can a daisy grow to as large as a dinner plate?
Yes. Dinner-plate sized daisies are called sunflowers.
