I originally tried 3/4". I did get a nice rounded arch, but it wasn't stable enough vertically. The electrical conduit is much more stable but the joints are weak, hence the 1/2" inside to strengthen the joints. One other item of note is that the couplers you use are important. The screw down version is much sturdier than the compression style. I plan on trying to bend new pipes next year in order to get them more rounded. A pipe bender isn't the easiest tool for bending across the entire length of pipe as it is intended to create one 90 degree bend.