The Big Picture skin itself is not actually modifiable [at least not as of yet] because everything is reset when the client starts.
The best option, AFAIK, would probably be to use some command-line options for Big Picture mode:
-480p - Run tenfoot in 480p rather than 1080p
-720p - Run tenfoot in 720p rather than 1080p
-bigpicture - Start in Steam Big Picture mode
-fulldesktopres - Run tenfoot in full desktop resolution rather than 1080p, overrides
-720p as well.
-nobigpicture - Start in regular mode (force Big Picture mode off)
-tenfoot - Start in Steam Big Picture mode
-windowed - Run Steam tenfoot mode in a window rather than a fullscreen borderless window
-bigpicture -fulldesktopres should work for you (or
-tenfoot instead of
-bigpicture). These can be added to a shortcuts 'Target' to make it easy -- otherwise you'll have to use the command-line and open
Steam.exe with the args you choose, which is what Mac is forced to do (
open -a Steam.app --args argsgohere), and possibly Linux too but I don't know because I don't know Linux well enough.
Also not sure if this will help with the notifications being massive; if not they may be part of an editable portion of the skin but I wouldn't count on it because BP has it's own overlay definition AFAIK.