I'm not exactly sure what you're looking for, but you can use 'xdotool windowresize 0xabcd 1920 1080' to resize an arbitrary window, 'xdotool windowmove 0xabcd 0 0' to move a window, and you can use 'xwininfo -root -tree' to get a list of all windows and the window ID which you would pass to xdotool.

If you want to write it yourself in C using Xlib, you can use the 'XMoveResizeWindow' function. You could use 'XGetWindowAttributes' to get the size of the root window.

I'm just not sure if you need a window manager to handle the dialog windows. If you do, I would just use Blackbox.