i don't have a set tank so I'll just give you a run down on how i did the majority of mine in regards to your questions
Size: I actually would use those 'betta' cube's - the ones that are 10x10 (so the wee little ones) as a/that's what I had haha (I actually used them to place the fish in while i did 100% water changes on their tanks... b/a smaller size gives them less room to move so not 'chasing' them around the tank trying to get a photo of them (good in theory but they still move pretty darn quick the little buggars)
Backing: Most of the time I never bothered with this. Once I did set up a black towel behind the cubes and that was great for really showing the colors off. - white would be another alternative as it's a light reflector so could help if lighting was a little on the poor side. With the black backdrop I had a sheet of white card set to the other side of where the mirror and light source was sot that the white card would reflect the light back onto the fish.... If I can find a photo of that set up on my other computer I'll post a picture for you
Lighting: As I did the majority of my stuff during water changes, I'd always do this outside on the veranda, so I'd get to use a lot of natural light (afternoon light - not harsh morning or mid day light...) As you need to have them well light for your camera to be able to focus and catch them as still as possible. Lighting in my opinion is the biggest thing to getting a nice clear/clean shoot you have to have good lighting for the camera (no matter what kind/type) to function at its best
Other things I have found..... Take lots of shoots. If you can, make sure you can move around the cube to take a shot from another side, as 99.99% of the time the damn fish will be on the one side that you can't take a photo from ...umm.... mirrors work well, a lot of mine where either with mirrors or one of the other boys just on the other side out of view
I thin that's about it - well other then know your camera and its settings... what kind of camera are you using.??