Umpire cap lengths are based on the number of stitches on the bill. For bases, wear either a 6 or 8-stitch. For the plate, while you can use the 6-stitch under most masks, the 4-stitch ofers the best "combo" of ease of taking off the mask and look. Go 2-stich for an old school plate look.
Umpire caps are fitted, stretch-to-fit (System5) and adjustable. Fitted caps offer the sharpest look and are best for taking off your mask while stretch-to-fit can offer the most comfort and wiggle room if unsure of your size.