most popular sizes are obviously the smaller ones since they are cheaper and I get orders for multiples of them such as the 8 x 10 the 8 x 12's 5 x 7's etc.
But if you're asking where I make most of my money, its from the larger sized prints. There arent a ton of labs that offer the big sizes so thats where you really separate yourself.
However if you're planing on reaching out to local photo studios or just photo studios who do wedding/baby/family portraits you should be fine with just the smaller sizes and you can outsource your larger sizes.