Ideally, your pants/belt line would rest along the center of the shield (+/- an inch or so). At a minimum, I would have 1"-2" of the shield below the top of your belt (with the shield comfortably resting above your leg), and the remaining shield (above the belt) to protect the stoma. This will allow the bag contents to flow down to the bottom without it being pinched off from your clothing. FYI, the large shield is better suited for taller/longer torso bodies with high stomas. Medium works for a large majority of people with an ostomy. The Small works great for those really low stomas at or below the pant/belt/waist line.
If you are still not sure which size is for you, I would start with the Medium size shield as it works for most people with an ostomy. Try it out with various clothing and situations. Then you can exchange for 1" shorter/taller shield if you feel it's necessary. Everyone's ostomy surgery and body shape is very unique. It's definitely a trial and error process. Not just buying our StomaShield, but any ostomy related product you try on.
StomaShield Belt Sizing Guide
Just like buying a belt for your pants, buy the smallest size you can fit into. 38", 48", or 58". Our new belt is elastic, so it will stretch some. If you are in between sizes, get the larger size to be on the safe side. It also can adjust down to any size with the entire outside of belt made of loop side velcro. For example, if you have a 40" waist it's best to get the 48" belt then simply pull to tighten to adjust down to get a custom fit.