Typical set up is a plate that screws into the bottom of your camera. Then the plate mounts to a head and the head mounts to the tripod (or monopod) legs. There is no universal size, although Arca-Swiss plate style is used by several brands. But, and this is where it gets fun, there is a difference in dimensions for the Arca-Swiss plates between manufacturers. RRS notes compatability issues in the third paragraph here. If you go with the screw-in clamp, this isn't an issue as it adjust to the plate dimensions as you screw it in. But this is an issue with a quick release clamp as it clamps to a specific dimension.

Several manufacturers have their own proprietary plate, such as Manfrotto and Gitzo. So, take the time to match the plate with the head.

BTW, I second the recommendation for RRS. I use a Markins head and RRS plates on Gitzo legs, but the Markins head is adjustable and I have it set to the RRS plate dimensions.