You need to look at what really matters and whether it's 60 gallon or 80 gallons don't mean squat and will make no difference in performance THERE IS NO DISCERNIBLE DIFFERENCE in performance between a 60 gallon tank and an 80 gallon, all other things being the same! What you need to consider is the compressor type and motor, in this case that is just a simple single stage pump of a very common model sold on home type compressors. The motor will be a 240 volt, most likely a Century or possibly an Emerson either of which is so-so quality. The point is don't pay much for this thing because it can be bought for as little as $400 new if it is the 3 1/2 HP 15 AMP version (usually mounted on a 60 gallon tank), that 3 1/2 HP can be listed as high as 6 to 7 HP "Peak" but if the motor is 15 AMPs the HP is about 3 1/2 and the "Peak" rating is just sales gimmick non-sense. These little single stage pumps have a comparatively short service life to a good two stage so if it has a lot of time on it then it may have reduced performance. Open the drain valve and check for rust colored water which would indicate internal tank rusting, if it rusts through and starts leaking it is scrap and CAN NOT be repaired! Check for excessive oil around the pump crankcase vent which would indicate pump wear and blow-by which are common ailments of a high time pump. Regardless of what kind of specs you find published by the manufacturer about the best you can expect from that little single stage pump is going to be about 10 to 11 CFM@90 PSI for the 15 AMP version and no more than 13 CFM for the larger versions, that's if and only if it is like new because these figures decline as pump time increases. Once again, look at what counts and DON'T be swayed by the size of the stinkin tank it simply will not make the thing perform any better in spite of the common myth!
BTW, since "Peak" HP has been discussed so much here in the past I thought it might be of interest that CH has recently started rating that 15 AMP motor at a much more realistic 3.2 HP instead of the BS 6 1/2 HP number!