males get 6-10 inches full grown, females get 8-12 inches. No turtles are necessarily "friendly" but some can be more passive than others. I have run across a few aggressive males in my lifetime.
Watch how much you feed your new RES baby.
I have two RES and 1 Mississippi Map. They eat more then my map turtle and they always beg for food. They're around the same age as my Map but are bigger in size. They are little pigs!!! So just be cautious of the amount you feed. She/he will most likely grow at a bit faster rate then your Map especially if she's a piggy.
A RES will grow very fast. They will reach 5 - 6 inches in not much longer than a year or so. Full grown takes a bit longer. We always find babies and then turn them into our outdoor pond area when they out grow their tanks. if you want water turtles that stay smaller, go for the painted turtles or the little mud/musk turtles. They stay around the 4 -5 inch mrk and grow uite a bit slower than the sliders. Still, sliders are so darn cute and funny to watch when they are little. Most of ours could be hand fed if you didn't mind a little nip or two. Wild adults can be mean but hand fed captive ones raely are.
One of my fav aquatic turtles....these are the clowns of the aquatic turtle world, they are also crazy eaters (worse than goldfish) and will try to convince you that you have totally forgotten to feed them---each and every time you go by their tank...I remove them from their tank each cleaning and if I would let them have a chance they would give me a nip...but don't feel like it is them being mean, just them being a turtle...lol...
There are some good sites also that offer up a variety of tank set up ideas, basking area ideas so they can come out of the water and dry their shell (important), heaters to use and temps....