26" S-7 wheels have a Bead Seat Diameter (BSD) of 571mm. The best tires for these are the Kenda 26 x 2 x 1 3/4" ones. S-7 wheels came on Schwinn's Middleweights, which are usually considered to be pretty common in a relative sort of way.
26" S-2 wheels have a Bead Seat Diameter of 559mm, and as previously stated you can use pretty much any decimal (1.5, 1.75, 1.95, 2.125) size tires on these rims. The old S-2 wheels are the same size as modern cruiser and mountain bike wheels. The S-2 wheels came on Schwinn's Heavyweights, and as most Schwinn Heavyweights stopped production in 1955-56 (only the Wasp and Phantom and Cycle Truck continued on) the Heavyweights are older and considered rarer and more desirable. And since some of Schwinn's more collectable HW models (Phanton, Panther) used chrome S-2 rims, these parts are always in demand.
And since it was thrown out there, a BSD of 597mm is the size of Schwinn's skinny 26" wheels, the S-5 and S-6.