i always consider how much media i want first. taking into account space needed for partitions, prefilters, heaters, drip plate, plumbing, and pump.
and as mentioned you need to be able to hold the water that will drain in power outage.
my advice is to go as big as you can fit, you will not regret it. more water translates to more stable conditions.
this is for a 100 gallon?
i wouldnt go smaller than the 29 for that tank.
one more thing to consider, the larger the sump the easier it is to put together, building small ones are like building a ship in a bottle hahaha
