I'm not sure exactly what you mean by Junior Olympic size, but it's pretty easy to convert if you know the following:
1760 yards = 1 mile
1600 meters = 1 mile
So if your pool is 25 yards long, you need to swim 72 lengths to do a mile (it's actually a tad over a mile--i.e., 1800 yards).
If your pool is 25 meters long, you need to swim 64 lengths to do a mile.
If it's some other weird distance, you'll have to do the math based on the above equivalents.
Hope this helps.