the biggest you can fit in the stand is a good start...
general rule of thumb is 1/3 the tank size...
75 gallon tank - 25 gallon sump
read that again.. 25 gallon sump... not a sump with 25 gallons of water in it... the main reason for upsizing a sump is for when the power goes out/pump dies the water that will drain from the tank will not overflow the sump..
do some searching.. theres 23985729875629875692875 threads about how sumps work/size/blahblahblah
ok thanx i have seen many threads on how to build a sump but none i found tell you how big it should be or how many gph it should be or infact how much bio matieral i have seen people use straws but somewhere i heard to use 2 liter bottle caps is this feasable.
you can use whatever you want for bio... the BB does not care what it grows on.. the trick is surface area.. the more surface area your media has the more BB it can hold.. plastic pot scrubbies (dollar store) are great and super cheap.. but like i said.. you can use whatever you want..