To size the gloves correctly, I believe you measure the width of your hand across the knuckles. The sizes go:
S/M = 3 3/8" knuckle width or smaller
M/L = 3 3/8" - 3 3/4" knuckle width
L/XL = 3 3/4" or bigger knuckle width
Knuckle width is the measurement across palm of hand.
Standard grappling/MMA gloves should be 4 oz. Too much bigger, and you're more looking at JKD or MT Clinch Gloves. Whatever you're looking at, I would definitely suggest that you make sure to get an open-palm style. There are a lot of wrist-wrap gloves that have the center piece to offer padding for striking, but completely take away from being able to grapple effectively.
I hope that helps.