By "Keys" I think you mean "wrenches" (aka "open end wrenches", or, as the Brits say, "spanners").  There are different sizes for the different stays and, yes, some are odd sizes, seeming neither standard metric nor SAE (inches).  When I can't find the right size, I use an adjustable wrench - and you need a large one for the backstay to get enough leverage.  You can also insert a large screwdriver in the slot of the turnbuckle bodies instead of using two wrenches.  You may want to wrap a cloth on the turnbuckle first to prevent marring the surface.

If you are buying new wrenches, I'd suggest a combination set of both metric and SAE sizes.

Good luck with it - Craig - s/v Sangaris SN#68