It really depends on how you want to go about it. If you want the greave (shin piece) to be one single item, then the only thing you can really do is make it big enough to put your foot in, and it should be about the right size, unless you have a really small foot, in which the rules are different. This will make it so the greave is quite loose on your leg, and will need padding to fix the problem.
However, if you are fine with splitting it apart, then you have more options. If you are one who can split it in half, then you are fully capable of making the greave small and snug on your leg. A big part of scaling is deciding on what method of putting on the suit pieces you intend to use.
Keep in mind that at first, it is common for suit pieces to appear too big when wearing them alone, but will look more normal when all of them are worn together. Also, just about every suit piece requires padding of some sort, as the padding is what allows for stability and comfort. Furthermore, lots of suit pieces aren't exactly anatomically correct, and will not be able to conform to the body in a snug manner. For me, my greaves were quite loose, and were stabilized by EVA foam and eggshell camping pads.