The missions used to be complete communities, with dozens of buildings. They were usually set up as a quadrangle. One of the few pictures every made of a historic quadrangle was take in 1875, showing San Buenaventura (in the town of Ventura).
The two mission that still have the most space around them (and have the most complete original complexes) are San Antonio de Padua (where the church and quadrangle were rebuilt) and La Purisima which is now a State Park. La Purisima has about 20 buildings and a coral with lots of animal.
If you have a specific mission in mind to visit you can get a good sense of what it looks like in one of the online tours that are available. Our key facts section for each mission shows a variety of images, and the slide show for each mission provides several additional images.