16mm is as wide as you can go because it already comes up onto the tire seat (raised side area) slightly. Any wider and it will interfere with the tire seating. Use 16mm, and stretch it down tight when applying.

One thing that interferes with tape sticking are oils and waxes left over from the manufacturing process. Use Ammonia (Windex) on a rag to thoroughly wipe down the inside of the rim and let dry before installing the tape. You can also use mineral spirits, alcohol, or acetone if you prefer, but not nail polish remover which leaves an oil residue.