Each brand fits differently. Trying it on in person is really the best option. BMW gear is really just Aerostich, so if it fits you, you can order an aerostich piece in the same size.
When it comes to pants, I find they are pretty close. Ditto for jackets. But you have to know your waist and chest measurements to use the sizing charts properly.
Boots are much harder. Each manufacturer uses a different foot mold. I find Icon fit wide but small, so they fit my overly wide feet nicely, I just have to wear a full size larger than normal. Joe Rocket and many of the other brands are simply too narrow for my foot.
The nice thing is, you don’t need to try the exact model you want, just the same company. All Joe Rocket boots have the same foot mold, so if a size 9 joe rocket boot fits, another style of joe rocket will still fit as a size 9.