A.We believe in the .700 size. It is the most used size by professionals and that size is the most "uniform" of all of the sizes. Also, consider the difference in volume that the piping can carry. If you take the .600od (.500id) and compare it to our .700od (.600id) our pipe can carry additional 44% more water! We can't understand why anyone would buy smaller pipe for a main line. Down the road, if you need additional fittings, or need to add to an odd-sized pipe, you might have a problem getting that oddball size. We say, stick to what most professionals buy. You'll be happier in the end.