Hi new to this site and have a question..

[MKLKT]

For a 90 gallon you'll probably want at least a 20 gallon sump in order to fit the required equipment. The more space you have to move around in the stand/sump area the better, otherwise it gets frustrating and overcrowded really quick. Remember that the sump is only about half-full, so you aren't actually adding 20 gallons to the system size.