Yes, wild caught isopods are perfectly fine as long as they are harvested from a location that you are 100% confident is safe. The size does not matter either; they all feed on decaying organic matter, fungi, etc. You'll most frequently find them in and around decaying wood that has been laying in the woods, usually in areas that stays fairly damp or wood that almost always stays saturated (they do have gills and require a high moisture content in the environment to thrive).
Isopods are crustaceans, so they differ a bit from the other arthropods we are used to feeding our reptiles. Similarly, they have a shell composed heavely of chitin, but if eaten they can provide a variety of unique micronutrients that would otherwise not be obtained from your traditional feeder insect. I wouldn't advocate feeding them as a primary food source, but if your herp happens to munch on one that is cruising around the viv it shouldn't be an issue.