Oh boy, I'm not very good at this but one thing that comes to mind first is how deep you NEED it to be. If you are in Ohio and want to overwinter your fish, you will want some depth to protect them from becoming fish-cicles in the winter. Maybe 3' deep at least, though others in your area can chime in to give a more accurate depth.

Here's where "Anne's Math" (which drives my mathmatical BF crazy) comes into play. Don't laugh if you are mathmatical; this is the best I can do.

If you choose 3' deep, then you have two 3' sides which = 6'; you also want at least a 1' overlap at the top of the pond, so that brings it up to 8' allocated - so 13 - 8 = a width of 5' for the pond.

The 5' width isn't allowing for plant shelves, so if you want them, then the pond will probably end up about 3.5 to 4' wide.

The length is figured the same way: Decide on how high you want your waterfall, let's say 2' (hmmm let's make that 3' because you will have "steps" in your waterfall which adds inches. You have the two 3' end sides plus 1 foot each end overlap plus the waterfall which equals 6' + 2' + 3' = 11, which leaves a length of 9'.

Still with me? I almost got lost there for a moment myself.

Okay... you want a kidney shaped pond. That will shorten the length quite a bit as you will have to make folds on the inner curve to make the kidney shape. Sorry, I just don't have a clue as to how to figure that one out, but I would guess, if I were doing it myself, that your pond might end up 5' long. Maybe shorter.

Hopefully someone else can do a better job helping you.

Anne

I agree with Anne. I do think she is probably a little pesimistic about the shortening effect of the kidney shape.
And the whole pond doesn't have to be 3' deep.

However, since you are doing this from liner to pond rather than; here's my hole, how much liner will I need; you can dig most of the pond and flop the liner in and see if you can go a little longer/bigger.