get a tank as large as you can fit under the 90 but making sure you can easily remove any equipment from it.If your having an in sump skimmer allow for the extra height as these need to be at a certain depth in the water to work( around 8") so if your sump will have 10" of water you'll need to raise the skimmer.3 section sumps are what most people seem to use.1st section water inlet,heaters,skimmer and uv.Then baffles.2nd section live sand/live rock and or macro algae,3rd section return pump section,try to keep this section quite large if you dont have an ATO( auto top off make up with ro/di water) otherwise can be smaller,This is due to the return pump section is the section which the water level will always alter due to evaporation.
