Tank size is relatively unimportant and so is max psi once it is "enough". The key factor is air flow, standard cubic-feet-per-minute (SCFM) but that spec can be hard to find on consumer-grade compressors. A hand bicycle pump can deliver 100+ psi, but it will take you all day to add a couple lbs to even a car tire.
The max regulated pressure needs to be higher than the pressure you need in the tire, at least 10 psi more and preferably 20+. Once that point is achieved, more psi doesn't help all that much. 125 psi is usually sufficient for all up to about 110 psi, which handles just about all RV tire needs.
SCFM varies inversely with the pressure, so you actually need a couple values for it to know what you are getting. In my opinion you want at least 1 cfm at the tire pressure you will be using, so 1-2 scfm @ 100 psi is a reasonable target. A higher SCFM means the tire can be inflated faster, but more SCFM requires more/better hardware and the price of the compressor jumps up quickly. Often the manufacturer quotes CFM at some low pressure, e.g. 40 psi. That's useful for car tires and air-driven hand tools, but not for large tires.
Tank size affects how often the compressor runs. A bigger tank helps deliver air without the compressor cycling on, so a little less noisy. Actually, less constant noise. 3-6 gallons is usually sufficiently convenient.
The popular 6 gallon Porter-Cable unit has a 150 psi max, 125 psi working pressure, and 2.6 SCFM @ 90 psi. It does a good job, but any another unit with similar specs would do as well.
