I would think that size upper reservoir would work fine if you are running the pump that frequently. I run 4 gallon buckets for the upper rez, but the lines plumbed into them reduces the volume somewhat. I don't usually go over 6 plants, but even when they are pretty big my rez only drops a few inches and it gets refilled twice a day. Runaways can drain it in a hurry, though.
I was just able to check, and the last adjustment seems to have done the trick. The pots I can reach are nice and heavy, and there doesn't seem to be any additional runoff in my trays I'm still going to put saucers under my pots though. I'll get the saucers and a pump later today. I had to order the bulkheads, so I wont get the dual res up this weekend. Probably just as well, the saucers will be a chore, all by themselves. On the bright side, I'll be able to accurately gauge the moisture content(weight) of each pot as I work.
I'm pretty sure I'll be ok with a 3.5 gal bucket. I'll be pumping 15min/hr. Unless there's a runaway, I can't imagine even 32 small/mid plants going through the whole 3gals in under an hour. The small upper will also help minimize waste in the event of a runaway(I hope). The 3.5 bucket, when full, will be light enough to hang from the roof of the tent without any reinforcing, which is a big plus. A 5gal bucket is just a little too tall for my needs.