All the snakes I have in the 11" range are females that are eating 2-3 fuzzy mice a week. What yours will eat depends on how heavy it is, and it's individual appetite. Some snakes never seem to eat very much, while others will eat everything put in front of them. Generally it is safe to "eyeball" it on the proper sized food. Next feeding, get a fuzzy and try to compare its width to the width of the widest part of your snake. If the fuzzy is about the same width, then see if your snake will eat it. (If the fuzzy is much wider, see if you can return it for store credit or something. If the fuzzy is much thinner than the snake, try to get a bigger fuzzy or a small weaned mouse, maybe even a pinky rat.) If it eats the fuzzy, you can try to give it 2 the next week, and 3 the time after that. I breed mice on a small scale and also have a decent sized stash of frozen mice, so I usually have all sizes available to me. If you only have 1 snake, it probably wouldn't be economical money or time wise to try this, so you might just have to deal with what the local stores have at the time. I'd recommend getting 1 more fuzzy than you think your snake will eat, and if the snake doesn't eat 1 or 2, you can just throw them in your freezer for the next week. This can help slowly adjust him/her to frozen food, might push your order into a quantity discount range, and supply food if you can't make it out some week. In short, at 11 inches, (without having something to judge the body of the snake) I'd say it's probably a good time to try moving up in feeding. My rule of thumb with feeding is that I give them as much as they will eat, and once they have taken 2-3 meals of 3 of a certain sized mouse, I move them up to the next size. Different people probably use different systems, so I'm not saying this is the best way, just what works for me. But at this point, I'd guess it's probably safe to jump from 2 pinkies a week to 1-3 fuzzies a week. Assuming it eats all its meals, in a year or so if it's around 20" and starting to pack on weight, you probably have a female; and if it's under 18" and still thin, you either have a male, or it needs some more food.