Depends on the jeans. I have found the most versatile width to be 1 3/8" for slim jeans and slim chinos but belts are quite hard to find in this width. 1 1/4" would be the narrowest I would go for slim dressy denim. 1 1/2" works better with with less dressy jeans and fits. 2" belts can work with workwear type looks but most jeans will not have wide enough belt loops to take them.