As stated in the manual, the following code will resize the window width to 50% of total screen width:
Resize 50 keep
But in case of a Xinerama setup, it seems that the percentage calculation is based on the “global” screen size, like the “@g” option in geometry strings. Is it possible to specify the screen as the base to calculate size percentage?
Make the window occupy the left half of the primary screen
AnimatedMove screen p 0 0 Resize 50 100 #(The window will resize to 50% of global screen width. Maybe something like "Resize screen p 50 100"?)