What would you estimate an average college graduating class size is if there are about 5,600 total undergrads?

Just divide by 4, and that will give you a rough estimate. Some people might graduate early; some might drop out, there may be administrative reasons why one year's class is larger than the next year's. But roughly, if there are 5600 undergraduates, then each graduating class will contain approximately 1400 students.

Since many freshman will drop out, I'd bet that the number about to graduate is about 1000.