they are generally smaller than males, but not by much. it does depend on the individual fish though, and i have had a female or two that was larger than any males i had at the time.
she looks to be at least 6 months old. with bettas, their fins are much shorter when they are young, even with females. if you got her from a store, i would say she's not younger than 6 months, and probably closer to a year. pet stores don't tend to sell the really young bettas because nobody wants to buy a fish that isn't yet fully finned or fully coloured up (juvenile fish often won't have their full colouring due to it making them an easier target for predators). your female is probably not full grown yet, but i don't think you'll see her get a whole lot bigger.
to help in not transferring snails over to this tank if you want to move plants - take plant out of tank and inspect for snails and eggs. ramshorn snails lay their eggs in little circles, mostly on the underside of plant leaves. they are easy to scrape off with your fingernail. pond snail eggs will be in a jelly 'lump' and are even easier to wipe off. you could give your plants a quick rinse in tapwater to help wash off itty bitty baby snails. won't hurt the plant any. and if you are paranoid about chlorine going into your tank, give the plants a dip in a container of water that you've dechlorinated after rinsing, before you put them in your betta tank.
and if a snail or two does get into your betta tank, it isn't the end of the world. snails can be beneficial, eating leftover food and rotting plant matter. they help to keep the tank healthy as long as their numbers don't get out of control.
