I have a 55 - 4' wide, 2' high, 13" deep. According to HelloLights, their retro t5's are 2" wide including housing - so my 12" hood may squeeze in 6 of them - and obviously would take 4 easily.
I'm not sure what I will try to keep eventually. I like zoos and what not. I'm told that I'll need more (like 6x T5's) for acropora and anenomes? I'm still getting familiar with what options there are.