It depends very much on the species and whether the tarantula in question is male or female. With most species, it's fairly simple to determine when a male matures because he changes physically (e.g. tibial hooks on the first pair of legs and bulbous shaped pedipalps). With females it's much more open to interpretation, I have seen people say a female is sexually mature when she is anywhere from 60–75% of their adult legspan. Do some research about the species in question to determine what their adult legspan is for males and females and then you'll have an estimate of when to expect them to mature.
I would like to know why you're thinking about breeding tarantulas if you don't know how to determine whether or not they're sexually mature, though.
i have been doing all the home home work i can and talking to as may ppl as i can. for my best gusse she would be but i am hoping to find 100% ish conformation before i try anything. i am verry cautious and maybe too cautious so i ask a lot of questions. i know the 2 rules of thumb are 2/3rds the size for a ready female and or the same size as the male. i have had this one for almost three years and she is a little over 5 inches. i am just being carefull, any helpful hints or suggestions i always listen.