The width should be roughly equal to the distance from the middle of your kneecap to the ground, or about 1 inch (2.5 cm) more than the width of your shoulders. It is the distance between the centers of both bindings and it simply tells you how far apart are your feet when you are strapped in.
This is why freeride boards have inserts setback and 100% freestyle snowboards do not. That is a very good idea in deep powder, not so useful in the park. Standing a bit more towards the back of the snowboard puts more weight on the tail and keeps the nose out of snow. Insert setback is very common and the amount of setback is related to the type of snowboard (freeride, freestyle…). In this case – even if you put your bindings in the center holes, they will be a little closer to the tail of the snowboard. That means they are not centered in the middle of the snowboard but set a little towards the tail. When you mount your bindings you need to know if the inserts already have some setback put in. Snowboard inserts are two metal pieces integrated into the snowboard that have several holes, which are used for mounting your bindings onto the snowboard. Are the inserts always in the middle of your board? Stance setback is the distance that tells us how far back the center between the bindings is from the center of the snowboard (actually from the effective edge center of the snowboard, nose and tail can be different length, but do not influence the effective edge center of the snowboard). Setback What is snowboard stance setback (offset)? Setback and angles are a bit exaggerated to be more visible. Here is a diagram that shows what each of the terms mean: Stance width, angles and setback together with snowboard foot placement and position. You can and should play a bit more with your stance angles to find out what feels best for you. Setback and width are usually set when you mount your bindings and then you usually leave them alone. There are three basic measurements you need to decide on when you are playing with your stance: