Pashuteh Yid-
The rule with lumber is that you take 1/2 inch off of the given size – a 2×4 is really a 1.5″ x 3.5″, a 4×6 is 3.5″ x 5.5″, etc.
If one of the pieces is a 1-inch measurement – i.e. 1×6 deck plank or 1×12 shelving pine – the 1 is 3/4 of an inch; a 1×6 is .75″ x 5.5″
The missing fraction is due to the finished size of the board – if you look at rough-cut unfinished studs in old houses you will find that the 2×4 is truly 2″ x 4″
The 2×4 or 2×3 that you support your schach on should be turned so that the narrow (1.5″) sides face up and down. This gives the board added strength and rigidity in the direction it will need to support the load.
The floor joists in your house are always positioned in that direction, and for that reason.
I read somewhere that doubling the width of a board doubles the load it can carry, but doubling the depth of the board exponentially increases it. I don’t know if I believe that (or even understand quite what it means), but it sounds good.
Sheetrock, plywood, particleboard, etc. is truly the size it’s sold as – usually 4′ x 8′.
Molding and wood rods also are sold under their true measurements.