It's a standard miniature scale reference. It is generally the height of the mini from the feet to the top of the head of a standard human character. Obviously, different characters are different sizes (eg, the Cragheart is a lot taller than the Mind Thief) so each mini will be different heights in real life but they conform to the 32mm scale.
Borrowing from this thread
, if you're going to replace the bases with something better/different/fancier, you need to know:
- the map hexes are 19.05 mm long on each side;
- the 'diameter' of each hex from corner to corner is 38.1 mm
- and the height of the hexagon from edge to edge is 33 mm.
So assuming a round base, it should be <= 33mm in diameter.