In general are most strat bodies like the ones out of kits, the same size as fenders etc

because ive seen one in a kit with a neck that i am considering

and a scratchplate from somewhere, which is for a strat but it goes no persific sizes
persific? but yes, strat bodies should be interchangeable.
'59 Bassman Reissue
Custom Strat
Custom McCarty
Custom OM
Big Muff Pi
Dunlop Wah
Anything thats strat licensed should work. Problem you will run into with pickguards is the screw holes. They have made many kinds over the years. Some have more screw holes than others. Even in the same year they can use different numbers of screws. Just one of those wierd things guitar companies do. And there is the 6 hole and 2 hole wammy bars. The six hole is what is considered vintage. But you can still find both kinds on new guitars. Dont try to save and buy the cheap one, its pretty critical to staying in tune. And the edge that bears against the pins or screws will wear over time.