Flowers are quite fascinating and there already exists a large variety of them. What we perceive as flowers may actually be quite different - the world of plants is quite varied.
For instance the above Caca Lilly is actually not a single flower, but a conglomerate of hundreds of flowers assembled together, with not a petal but a bract (broad leaf) that is white. This allows pollinators to be attracted similar to smaller flowers, but allows the broad leaf to gain in size.
It is also not unheard of for leaves to be large - therefore it is possible indeed for flowers to grow in size commensurately if the evolutionary pressures are present to do so.
The world of plants is fascinating - take for instance Pando - a plant in Utah. Using one root system it has grown to the size of 110 hectares, and is predicted to weigh 6,000,000 kg. This is the largest and heaviest organism on Earth - although on the surface it is often mistaken for a grove of Aspen.
It is easy then to conceive of a flowering system being very large - however considering that flowers are for attracting pollinators and consume resources to do so, the greatest limitation to their size is probably that they don't need to be large to attract the insects they need to attract. If there is no evolutionary pressure to be large, then there may be no need for it to be so.