There's no small, medium, or large. Thobe sizing runs on two things: a length number and a width letter.
The number is your length in inches, measured from the back of the collar to the hem. It's the part to get right. Too long and it drags. Too short and it looks borrowed. Between 5'6" and 5'8", go 56". Between 5'8" and 5'10", go 58". Between 5'10" and 6'0", go 60". Between 6'0" and 6'3", go 62". Right on the border? Shorter sits cleaner and more modern, longer is more traditional. Both are correct.
The letter is your shirt size. M, L, or XL. A 58L is a 58" length in a large. Between sizes, size up. A bit of room moves better than a tight chest, especially in sujood.
Full measurements are on the Fit Guide. Still not sure? Email info@thobestudio.co.uk with your height, shirt size and weight and we'll tell you exactly what to order.