I went to my rear passenget bleeder scew and tried to crack it loose. A 10 mm is too large. A 9 mm is too small. I tried a 3/8 box wrench and it seemed to fit but with a little pressure trying to loosen the screw it spun. I may have to buy new screws and remove old with vice grips. The nut does't look stripped. Anybody know the size?

 

5/16 and 3/8 are most common SAE sizes but depends on the year, later models started going to metric

Sockets often don't work on bleeders, have to use open or box end wrench, boxed is BETTER, or flare wrench end

Yes, use anything you can to get bleeder out and replace it
Remove cap on master and remove old bleeder, it will Gravity bleed out fluid while its out, then put new one in, it won't get air in it this way
You shouldn't have to remove the master's cap since its vented, lol, but I always do

And wrench/socket sizes are NOT always exact, the more expensive sets are usually correct, but as the price goes down so the accuracy in the size it works on