One of the main concerns was the small size and the ability to easy install and replace avionics. This led to the decision that all external side panels will have to accommodate being taken on and off - no welding, only on the main structure.
This is a technique known to aerospace engineers for fifty odd years.
The timing of it's original discovery and implementation had a unexpected impact on space history though... NASA first encountered the same problems with Mercury - not so much because it's size, but because all the systems were packed inside one on top of each other with no provision for access. This caused many problems during assembly and launch preps as often connections had to be broken and unrelated equipment removed to get at a part that needed replacement, repair, or adjustment. So, when NASA and McDonnell (they hadn't yet merged with Douglas) were evolving the design into the Mercury MKII and eventually Gemini, they re-arranged things. They shrunk the pressure vessel a bit, enlarged the structural shell a bit, and packed as many systems as possible into the space between and behind access doors.
But Apollo's design was already largely frozen - it retained the Mercury type design of having almost everything packed into the pressure vessel. (Yes, the design sequence goes Mercury-Apollo-Gemini, out of order from the flight order.) The result was that it was extremely difficult to work inside the Apollo capsule, to track work accomplished, and easy to damage adjacent systems and equipment - damage that was later believed to have been the source of ignition for Apollo 1.