Nozzle sizes will be different for mini or full guns.
in a mini gun a .8 or 1.0 will do it all.
1.0 if youre on the edge of needing full size gun, like doing doors, .8 if youre not going much bigger than wheels.
What size objects do you intend to paint?
A full sized gun is really only going to be necessary for something 2k urethane or lacquer paint and only for something maybe hood (bonnet) sized or larger.
A mini gun will do anything smaller or even large if your using some slower flashing paints like enamels.
Compressor wise , on a 25 gallon ( around 4 CFM at 90psi or 6 cfm at 40 psi) compressor you'll be ok with a mini gun.
Full size guns on a 25 gallon compressor there are only two i Know of that can get away with it.
Sprayit sp33000
campbell hausfield DH580000AV
They are full sized guns that get away with very low air consumption, theyre cheap as hell but none of the better quality guns can run so low of air volume. I use them when I have something bigger to do and have to be mobile. I can get fair results from them with the fluid control restricted if its not too hot out so the paint doesnt flash quickly.
Dialed in and under the right conditions they work pretty well.
You will still have to wait on air a lot of times/constantly fiddle with the regulator because for example as your supply pressure goes down from 135 psi to 70 with the regulator set to 20 it will drop from 20 to 15.
If you can move up to 30 gallon (6 cfm at 90 psi 7-8cfm at 40psi) Then these guns will be okay to run.
Rule of thumb is try to get as close as you can to doubling your volume of consumption wit your compressor but at minimum be over it.
Ill leave the unit conversions to you