Small medium large and Opra size ( A - B - C -D -E)
Pick the size you think you need and go up one.
Never seen the E size in real life before
If cost is not a real priority to you and you like the idea of a fully adjustable mount
I think the RAM mounts are worth every penny.
These are metal and well made. I personally enlarge the mounting holes very slightly from
6-mm to 1/4 inch and buy stainless steel hardware ( lock nuts Allen heads ect)
A = size mount a phone or handheld thing .... B or better yet C mounts a small 5" or so fish finder... gps D = 7" or larger unit .. Some say overbuilt / overkill !? I say yeah , your honor I rest my case