As an Electrician by trade, A 100 amp panel should be plenty. You can get one with 20 or 30 slots. If your shop is 1,000 square feet or more I would recommend a panel with 30 slots, otherwise 20 would be fine. The size of your subpanel would also depend on the size of your main panel. You can't really have a subpanel the same size or bigger than the main panel that is feeding it.