The narrower 83mm Monster allows for simpler turn initiation and a quick transition from edge to edge making it a good groomer ski.
The 88mm Monster has a wider profile for stability and increased flotation. The 88 is a versatile option that can maneuver through trees, charge through crud and still hold an edge on groomers.
The lengths are from tip to tail of each ski. Shorter skis turn tighter and are easier to control, but will not float or be as stable at high speeds as a longer ski. Longer skis will allow you to go faster, but some quick turning ability is sacrificed.