I use a full size iPad (in a PA28) and space isn't an issue for me, and I like the size.
It probably is more of an issue if you fly something with a stick.
I use
EasyVFR by PocketFMS (I'm part of the beta team). Yes, it's possible to easily replan flights in the air. The interface for EasyVFR was specifically designed to make this as easy as possible. A no point in the re-planning process is a very accurate touch on the screen required, and there are no time sensitive parts to the planning.
So vibration or turbulence isn't really too much of an issue, and you can stop what you're doing to look outside/talk to ATC without losing your place in the re-planning.
Unlimited FREE electronic flight plan filing is included with a Frequent Flyer membership, though you'll probably struggle to get a data connection while flying, so this will probably have to be done on the ground
