Chilean Rose Hairs top out around 5-6 inches or so leg span. If yours around that size, its probably an adult.
Sexing a tarantula is not really an easy task for the beginner... you'd probably be best taking it to someone who knew tarantulas and get their opinion. But besides that, take a look at
The Tarantula Stop. The males will have a set of 'palpal bulbs' on the second set of legs. And males generally are skinnier than females (of course without a frame of reference this is hard to see). There are probably plenty other sites out there on it if you look.
Rav
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