On the external monitor subject. Not only size, but resolution is also quite important. No point in a 27in. screen with 1280x1024 resolution unless you are sitting across the room.
ALthough I haven't done any CAD is several years, I've found just over 100dpi (dots-per-inch) to be just acceptable. My current monitor for mostly text and some graphics, is 23inch at 104dpi, that's 2048x1152 pixels. Sitting with my eye socket 24 inches away, I can scan the screen rather easily with very little head movement, of course looking at details requires head twisting.
I'm likely several decades older than you are, so your eyes may be able to take advantage of a greater dpi... if you can find and afford them! Also make sure the computer can handle the higher resolution.
Remember that your visual system is a parallel input channel, you don't want to be constantly scrolling trying to get 'the big picture' of your project.
Cheers,
Tom