Usually either 7/8 or 1”. Buy a cheapie $29 digital caliper from Harbor Freight and measure it! You will probly find other uses for it..
You should verify the magnets won’t interfere with the internal compass (it will) or your binacle compass, if near. I’d avoid magnets near anything having to do with navigation.Im going to print out a plate to insert magnets in and glue it onto the base for a secondary means to keep it in place. I've been sailing in 15+ knots all day and it isn't moving but if its hit it will come off as it is.