the width and diameter of rim. if you can match those up to a tire. going with a smaller size or larger diameter tire should not hurt things much. if you have a 2WD tractor.
as far as trying to find tires. it normally takes opening the old phone book, to "tire" for regular automotive / car / truck places. it may take calling a few dozen places (not kidding) but normally after a few calls, someone may know of a place and be ready to jot down a name or a phone number. out of a few dozen regular car / tire places there may only be 1 or 2 places that may actual handle tires for tractors in larger sizes.
most places here in the US at least around me, may be a tractor dealer but do not handle tires directly. as a result use some third party company to buy and put on new tires.
you may try to do a internet search for R1 or R3 tires. to find nearby locations that deal with tractor tires. and once at there website or place, look up tires for the front.
keep in mind what ever you get, make sure the tires can handle the "pressure" when you have a fully loaded FEL (front end loader) bucket. or you may end up poping the tires off the rim. or bulging out the side walls of the tire.
the smaller fronts for a 555C once you get tires, any nearby regular automotive store should be able to place new tires on the rims for you. (tires should be small enough for most machines that need to deal with the larger size regular truck tires.)
