What percentage of a wetland must be a certain class type in order to be called that class type? (emergent, scrub-shrub, forested)? I can only find "majority" of vegetation within the Cowardin Classification document. Any references/input would be helpful. Thanks.

Many wetlands are not all one type. Look at the National Wetland Inventory maps. Many wetlands transition from open water to emergent to scrub shrub to forest to upland. As far as how big to delineate these, it depends on the scale of the map. If you are mapping a 100 acre wetland, calling out wetland less than an acre too fine of detail.