Assuming what I call "random VFR bimbling" (meant in nicest possible way):
Is a 7'' screen so much better than a 4.2'' or a 3.8'' ?
For the "printed" VFR charts, very much so.
At what size does a handheld start to become too bulky?
Depends on your cockpit; I think a 12" screen tablet format is too big. 7" is ideal.
Ability to fix safely in cockpit?
Depends on whether you do aeros
I rest it on my knees when VFR.
How important are the different screen resolutions and refresh rates?
Not very; a hi res screen means the chart is readable in a smaller zoom but if you are 45+ you will need reading glasses to see it. (IFR approach plates are a different thing)
What do you view as crucial connections?
Route upload from a flight planning program e.g. Navbox or Flitestar VFR
What do you view as crucial battery life?
Longer than your longest possible flight there and back; otherwise you need to plug it in.
Types of GPS receivers?
Not really important.
If you can't connect to the internal electrics do you need supplementary batteries and what are the best sort?
Are all touch screens the same?
Advantages / disadvantages CAA map loaded compared vector map?
The CAA map is vastly better.