Medium plates for medium size vest, large plates for large size vest.
I've got a fake SAPI filled with sand that WEIGHS as much as a real one
I'm a small guy with no beer belly so I found them to be uncomfortable, but I cut out a plate shape from a 1/2" foam matress and it works great.
And to answer Aquamarine:
It adds alot of rigidity to your vest, keeps your mags from slapping your gut while running (great thing when you have 12), it absorbs alot of fall damage and prevents things like the end of a transmission from punching a hole in your ribcage when you fall on it