Round magnets are available in sizes from 1 1/4" to 3" (prices shown are for the 2 1/4" diameter) and the squares are 2" by 2".
The same selection of designs is also available on pinback buttons in a similar range of sizes.
But while you're looking at magnets, don't forget to think about the possibilities of my square celtic border section magnets, or my celtic number magnets.
Each one of those gives you some bits and pieces with which you can place decorative numbers on anything made of metal, or build up your own knotwork border out of a set of modular pieces.