Well for starters if you get a deep freeze in the Winter you would need to know how far down your frostline is plan accordingly. (what is the winter like in Arkansas). Regardless of whether you keep them out all year or not you will need to make the pond at least 3 ft deep. This way your turtles can escape predation.
Also you will need to have a wall around the pond directly and provide an internal basking site in the pond or a barrier (that the turtles can't climb (they can scale chicken wire and hardware cloth better as well as any lizard!!) around the pond so that they can climb out to bask on the ground but not wander away (they will wander away if given the opportunity).
The pond can be as large as you like it to be. Two turtles don't require that much area but you do have to make it deep. If you put in a kidney shaped pond that was say 3 feet deep and 6 feet long or longer that would be fine. The sky is the limit really.
Why not consider getting a pondliner and that way you can make a free form pond and do some landscaping etc. Email me privately and I can send you a few pix of my large turtle pond if you want some ideas.
Steve