How big can I go while keeping the same light?

[OCDP]

Okay, perhaps scratch the cube idea .... grr. I am being informed that angels need lots of swimming space (which I didn't know) .. so I am being advised to do a longer tank. I'm having a hard time figuring out what to do here... size is an issue. Length wouldn't be good for two reasons A) I have a modded PVC light fixture made that is only as wide as my 20g tank stand. And B) I don't really have that much space in my bedroom.

Now.. I do want to make the tank more suitable for the Flame... but I just don't know what to do. I'm pretty confused, I am very unfamiliar with tank dimensions and sizes.. I really just want something pretty basic and standard, I'm not too picky. I just need to make the tank a little longer now to make more swimming room for the Flame..

I need some help! haha, thank you.

Scott