I'd agree with Robin. Start with a 2.25" or even 2". That will be a belt that can be used in conjunction with the shorter casual kilts as well as with the longer traditional style kilts (though the 2" would be a tad on the narrow side for the traditional).

I believe though that as the length of the kilt increases the width of the belt should increase in order to maintain a visual balance in proportions.

When I wear my casual kilts which are 22", I wear a 1.75" or 2" belt. When I go to my Traditional kilt (24" + ), I use the 2.5" or wider belt.

These are my personal "guidlines" that I believe help keep the outfit looking neat and balanced.