I am currently using a MIX of planets from Fleetdock13, John Stone's sites, STOCK SFC3 planets and Stock OP planets with a few retextures on them. So I would say it is possible to get a range of planets from half to probably twice the size of a stock OP planet.
I have an interesting "ringed earth" that uses the stock model with one of John Stone's ocean planets.
DarkMatrix once did some planet texture extractions for eith Klingon Academy or BC, I don'e exactly recall which...
Noteworthy Planets I am using
PlanetEarth = Fleetdock13, Earth with Moon
PlanetWater = Terraformed ocean covered Mars from SFC1 days [no readme file]
PlanetRomulus = SFC3 Stock
PlanetQoNos = SFC3 Stock
I am also using the SFC3 SPACE MOD file for the backgrounds and I have to say it cranks up OP a notch or two visually IMHO.
Qapla!
KF