On an Earth analog, assuming all the selective pressures necessary are there, how large can a flower's bloom evolve to be? I'm referring the the vegetative part of the flower, the petals! I have no clue how large a bloom could become since different species have varying petal strengths, of which I can't find any information on.
The largest bloom to evolve here on Earth is Rafflesia arnoldii, which can grow to be 3 feet across:
Plants can get pretty big. The largest leaf in the world belongs to the Raphia regalis tree, and it can grow up to 80 feet long. Flower petals are basically just brightly-colored, specialized leaves; assuming a circular pattern, a flower with petals this size would be over 150 feet across. Two of those flowers would barely fit on a football field.
However, depending on the nature of your story, you should also address the issue of why such a massive flower would evolve that way. A flower's primary function is to attract pollenators, and the only part of it that's useful to a pollenator is the center. If your world is high fantasy or soft sci-fi, "a wizard/space-botanist did it" might be sufficient. If not, you'll need a reason for this massive flower's existence.
I don't think there is a hard limit; in theory with a stiff enough cell structure and a thick enough petal said petal could be just as big as you like. I mean there's probably a practical limit dictated by the cellular termination cycle of the plant, the flower simply can't grow past X size because the cells die before it can get that big but otherwise the sky's the limit.
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.
