Off-the-rack or off-the-peg kilt jackets are sold in the same sizes as suit and sport coats the world over. If you know your size in normal jacket you will be pretty close to the same size in a kilt jacket.

One caution though, kilt jackets tend to be cut narrower than N. American jackets. You may find that you will feel more comfortable in one size larger in the chest.

But if you now wear a Short, Regular, or Long jacket the same applies for a kilt jacket.

If you are worried about the length of the bottom of the jacket if you try one on without your kilt simply reach around in back and the hem of the jacket should fall at the crest of your buttocks or at the bottom of the Fell of your kilt.