People offer different sizes of mealworm like 1/4 in, 1/2 in, and 1 in. I was wondering which size is best?I would be wary of super worms, they are often treated with insect hormone mimics so that they grow to be extra large larvae. If that is the case, they won't pupate or become adults and therefore won't be useful for starting a colony. However, people have bred varieties of mealworms that grow really big without the hormones, but just make sure to have a reputable seller and ask questions if you are looking at superworms.
I'm not 100% sure that I am understanding your question, but the starting size of larvae doesn't make too much of a difference. The large larvae are usually what is available for sale, since that is what most people feed to their reptiles and chickens. Large larvae shouldn't take much additional time to pupate. Sometimes you can also find adults for sale, which further decreases the time it will take you to breed more worms.