You can calculate the required pulley size from first principles. A typical engine speed for an 18" mower is 3,000 rpm, though they will mow satisfactorily down to 2,000 rpm. You have a 28" mower, so you want to run your cutter at less than that - 18/28ths of that in fact. To get that speed reduction you want a crankshaft pulley that is 64% of the size of your cutter pulley. You haven't told us the size of your cutter pulley, so you should measure it and either tell us, or just multiply it by .64 and go find a pulley.
