Best bang for the buck: 10.5" iPad Pro
Best compromise between portability and screen size: 11" iPad Pro
Biggest screen: 12.9" iPad Pro (of course)
10.5" iPad Pro has a screen resolution of 2224x1668, which means for 16:9 material, it is 2224x1251 = 2.78 Megapixels.
11" iPad Pro has a screen resolution of 2388x1668, which means for 16:9 material, it is 2388x1343 = 3.21 Megapixels.
That means that for 16:9 material, the 11" screen is 15% larger than the 10.5". The two iPad Pros are the same weight.
OTOH, for 4:3 material (like old TV shows), they will show the exact same image size, at 2224x1668 = 3.71 Megapixels.
EDIT:
To further expand on this:
What this means is that for 16:9 material, both the 10.5" and the 11" will have black bars on the top and bottom, but the black bars will be shorter on the 11". OTOH, for 4:3 material, the 10.5" will be full screen, whereas the 11" will have black bars on each side.