I don't mean that a big tank is bad, at least as long as the pump and motor are up to the task. In the CFM range you are looking an 80 gal is about standard and will do nicely. What I meant was don't pass up a good CFM rating for a bigger tank and if you are choosing between two units of the same CFM then one with a 60 gal instead of 80 will take up slightly less room and be somewhat cheaper but the 80 will offer a slight advantage in power consumption and compressor life on a high output unit due to fewer start-up cycles but this is only a VERY slight advantage. Look at the CFM rating and let that be your guide since CFM is what runs your tools and if the CFM is not there then tank size will not matter anyway, rest assured that the companies you are looking at here have well engineered units that will be the right balance of HP/pump displacement/tank capacity and will serve you well just as they are and adding more tank capacity is unnecessary. The tank has no effect on the CFM rating and in fact we have one set-up for service trucks that puts out over 50 CFM@90 PSI and it only has a 20 gal tank because of space limitations. It is easy to find 80 gal units that have LESS CFM than some 60 gal models and that is my main point, I have seen people buy one with a bigger tank over a model with a smaller tank but MORE CFM thinking they were getting a bigger compressor because of the bigger tank when actually it is the other way around, it happens all to often and is a BIG mistake!
BTW that 7.5 HP Quincy is a really good outfit and unless you are going to run a big shop with multiple tools running at the same time it will not disappoint you. Take care of it and unless you just outgrow it you probably will still have it 30 years from now, those Quincys are that good!