How do I determine which wax ring to use under my toilet?
How tall the ring is depends on how close the toilet's outlet horn is to the flange. Under ideal conditions, the standard ring is just a bit too tall so that it compresses just enough to form a good seal without squeezing out into the drainage path and causing an obstruction.

Toilets with more than normal spacing will need a taller ring to achieve a proper seal. But if you use a tall ring without the flange on a properly spaced toilet installation, excess wax will be squeezed into the drainage path, causing an undesirable obstruction. This is why double height rings are often sold with flanges, the flange keeps the wax out of the drainage path.

But the flange itself is a bit smaller than the outlet pipe, so it forms a bit of an obstruction itself. Thus, it's not something you want to use unless there's a good chance you'll need it.

You can estimate which height ring is needed and if a flange is a good idea or not by placing a straight edge along the base of the inverted toilet and measuring from the straight edge to the ceiling of the toilet base. Adjust this measurement by how high or low the floor flange surface is from finished floor. The wax ring should be compressed around 1/2 to 3/4 it's initial height once installed. If it will compress less, you need a taller ring. If it will compress more, consider getting one with a flange to limit wax extrusion into the drainage path.