We recommend at least a 25mm tire for the Tubulars.
For the road clinchers, we recommend 23mm or 25mm for minimal rolling resistance, though for cyclocross applications, up to a 35mm can be mounted.
The M3 and M5 have a wide profile rim, so 25c's fit great. A lot of riders are starting to use 25c tires exclusively, some studies show there is no less rolling resistance on a 23c than a 25c. You can use the more forgiving tire and you'll fatigue less in the saddle from getting rattled around.
As far as PSI goes, we suggest using 95-100 PSI. Anything higher only gives you the illusion of going faster, because you can feel every grain of sand your bike is rolling over. But in reality you're not gaining any efficiency with a marble-hard tire.
For our X3 XC wheels, the max tire width would be 2.35. Anything larger than that will give the tire too much of a "light bulb" profile. 2.1, or 2.2 is a great XC width for those wheels.
The Enduro width rims love wide tires. We have a set with 2.75 honey badger DH tires and they rip on trail (and for snow riding). Our mountain Enduro rims can go mid-fat, up to 3.0 inch tire.
For X1 and X3 Plus, the max tire width would be 3.0