Seat harnesses and waist harnesses each fit differently. The first step to getting the correct fit harness is to determine which you prefer or which fits your riding style/skill level. In Jeff's video Waist vs. Seat Harness he discusses the different hook points, movement allowed by both models and briefly mentions bar size options. Ultimately, your choice in harness style comes down to what you find most comfortable for your style of ridding. When you take a lesson, ask your instructor to let you test out a couple harnesses or request a demo to try during your next session.
One of the main differences between a waist harness and a seat harness is that waist harnesses don't have leg straps. A waist harness fits around your upper waist, right around your center, and just under your rib cage rather than around your hips as a seat harness does. Most harnesses have an elastic velcro waist band you stretch over your stomach to get a tight fit. If you have selected the right size harness and the elastic belt is properly tightened, you should have at least a 1 to 1 1/2 inch overlap with the harness's belt end and where your bar lines up.
The next thing to check is for adjustability. The right fitting harness will leave you with some room for adjustment. This is particularly important if you use the same harness during winter riding with a wet or dry suit and in the summer with just a rash guard. If you don't have adjustability, you'll need to try a different harness size.
Unlike a waist harness, seat harnesses come with leg straps and sit lower on the body around the hips. There are two main types of seat harnesses: the traditional seat harness and a boardshort harness. The boardshort harness has a lot less structure and more casual look than a traditional seat harness.
When fitting a seat harness, the padded harness sides should not be touching or overlapping (about a 3 inch gap is good). The harness should have a distance that allows about 1 1/2 to 2 inch overlap with the bar and the right side of the padded harness.
The next thing to check is for adjustability. The right fitting harness will leave you with some room for adjustment. This is particularly important if you use the same harness during winter riding with a wet or dry suit and in the summer with just a rash guard. If you don't have adjustability, you'll need to try a different harness size.