For traditional kilt belts, 2.25" is the most common.

In the 2" range there is just such a limited selection of buckles to choose from so for selection I sell more 2.5" than anything.

To further analyze the issue, you can look at the style of kilt as a factor. For low waisted casual kilts you can get by with a 1.75" belt as the visual proportions are kept in balance. Once you go to a kilt with a high waist I feel you really need to wear a wider belt (no less than 2").