The main problem with any kind of mount is restricting the view of the instruments and the outside scan. I know to my cost the lack of scan of instruments and some of the installations I've seen with cables festooned everywhere is downright dangerous. Carefully think out what you actually need and panel mount if at all possible, hardwired in. The Garmin AERA 660 is an excellent example of a removable panel mount tablet.
I use a 7-inch tablet on a kneeboard, the biggest I think is usable safely in a small cockpit. It has enough charge to last all day without an external power supply and the internal GPS aerial works well. Occasional glance down at this to see how your heading is matching up to the track is all that should be needed in flight.
You certainly shouldn't be mounting anything that restricts view out of the window. Certificated aircraft like the PA28 and C152 are approved with the windows as designed. Think what the insurance company would do to you if it turned out you had an accident where they are able to say your view out was restricted or the cockpit environment was compromised by non-standard kit.
There was a ditching off Alderney a few years ago where the occupant in the right seat of a PA28 (who survived) said the last they saw of the left seat occupant was them struggling in a tangle of wires...
Small aircraft windows are made out of Perspex or similar, not glass. They are much more prone to scratching and other damage.
TOO