Sorry I have not checked in during the last 11 hours to answer your urgent question. (Ok, so it's a week!) You'll need to post the dimensions of the tank you want to build before I or anyone else could tell you how much acrylic you would need, but I'll guess that it sounds like 1/2" might be the right thickness. If you've never built anything out of acrylic, this might not be the best project to start with.

Possible dimension for a 90 gallon (approximately) would be 36" by 24" by 24", and I think the standard 90 tall glass tanks are 48 by 18 by 24, something like that.