Size really depends on your other technology. I have the small Apple iPhone and a laptop so I didn't want a big iPad so I bought the iPad mini. I use it and Sky Safari with a Skyfi interface to run the GOTO on my mount.
My polarfleece jacket has xtra large pockets. The iPad mini fits into theses pockets.
There are a number of other really useful apps.
Time by Emerald bounces signals off several network time servers and gives time accurate to about 1ms with a display and audible beeping.
Spyglass is a digital compass. It has true bearing readouts and will superimpose the compass protractor over the camera image.
There are also clinometer programs and numerous other apps.
Physical size is really personal preference.
Make sure you buy enough memory for all your apps.
There are models with wifi only or models with their own built in mobile broadband sim card slot. This adds a fair bit to the cost. I bought one with that option and now regret it because I seem have a wifi signal from another source everywhere I use it.
Joe
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