I frequently handle 24-26 lbs of grain in my 10g cooler (in fact, I've got 24.81 lbs in it right now about 30 minutes through mashing). 26 really starts to push it.
So 4/5 of that leaves you about 20 lbs to work with.
This is for 15 gallon batches, though, so if you're doing 5 gallons, you should have plenty of space.