I use a 10 cm glass cube. I used to use beanie boxes until they got too scratched.
I'm going to join the chorus of "lighting, lighting, lighting". You can improve the results of any camera simply by providing more light. If you're restricted to the in-built flash, reflection will be a problem however if you get close enough to the glass you might be able to get around it. With that in mind, before you decide what size photo tank to make, first get your camera manual out and work out the closest distance you can get to the subject. Then build the tank so that it's about 1 cm more than that with the view of getting the fish to flare against the back wall of the tank.
My point and shoot camera has a hotshoe so I bought an external flash and cable. This allows so much more flexiblity. I still try to get as much light on the fish using desk lamps but that's mostly because my non-DSLR finds it difficult to focus in the dark.