More than likely red ear sliders, they'll get anywhere from 10-16", but that'll take quiet some time. A red ear will generally hit about 5" at around 2years old, could be sooner than that if given the proper care.
Don't forget that they do need UVA and UVB lighting aswell as a heat lamp for basking, Calcium supplements are a good idea, another thing when the turtle gets older that you may want to invest in are cuttle bones, which is basically a calcium bone for the turtle to chew on.
Don't forget to add a variety to their diet, lettuce (roman) is recommended, but not all the time, duckweed and other aqautic plants would be munched on by the turtle aswell, and it's a good thing too. Some people may not agree, but live feeder fish (guppies and red rosy minnows) are reccomended, it gives the turtle exercise and it also gives them a nice chance to hunt just like they were in the wild.
Minimum tank for 1 full grown turtle is a 72-75 gallon, although the larger you go the better it will be, not only for the turtle to have space, but for the water quality.
www.redearslider.com, a great site with great information on these little beauties
I myself just picked one up, and this site has helped a bit.