Playing with tank dimensions. Need some help.

[OCDP]

I am no good with tank dimensions or planning them out as I don't have barely any experience with it. I do know I want a cube.

It has to be a size that can be lit only by 250w HQI . So depth and what not will come into play. I don't want the light to lose much intensity once it reaches lower in the tank. Seeing as I WILL have a carpet anemone, the intensity needs to be good so it will reach the anemone. I think length should stay the same as my current 20g (which I forget that dimension .. crap) Anyone know the size for a standard 20g tank?


Sooo.... has to be the same length as my 20g. (so I can use my current stand.. )

Any help is much appreciated. Thanks.