They do vary in size but the common size is 400mm in length with a 70mm, 80mm or 100mm diameter on the mixing component and typically an 8mm width. The common chuck size for most drills is 10mm or 13mm so a mixing paddle will fit both these common sizes.
I bought this 70mm Toolty Helix Mixing Paddle from Amazon so I could mix smaller one litre tins of paint as a 100mm mixer – or bigger – is too large to fit.
You can also buy flat bottomed paddle mixers that rest on the bottom of a container to help stabilise the mixer. But personally I prefer the mixer style I choose as I think it mixes paint better.