I have kept many different Ricordea florida and Ricordea yuma before... the largest one I have kept is about 4'' across and the largest that I have ever seen was about 10" and that was at the GA aquarium and is abnormally big for a Ricordea mushroom. They are relatively easy corals to keep but are not as hardy as other mushroom corals.
Depending on whether or not you bought them tank-raised or wild-caught will have an impact on their survivability and their lighting requirements. Wild-caught Ricordea tend to not mind high light situations as opposed to tank-raised which prefer Power Compact lighting or indirect Metal Halide lighting (ie: shaded under a rock ledge or very low in the tank. I have seen a wild-caught Ricordea yuma be fully open and happy directly under a 250W Metal Halide and I've seen tank-raised Ricordea melt under similar conditions. It helps to know where the specimen in question is from so you can provide it with the ideal captive environment.
As far as feeding goes, I have never noticed any real benefit from feeding Ricordea mushrooms. They get most of their sustenance from photosynthesis. However, larger Ricordea will eat small bits of food if it is placed near their mouth. Chopped up brine, mysis, or clam will suit it just fine.
And lastly, as far as growth is concerned, I have never noticed any real growth in any of my Ricordea mushrooms. I have had some for over a year and have noticed little to no growth. It is extremely difficult to tell because the coral will expand and shrink periodically depending on its environment.
Hope this has been helpful.
-Chris