Resizing windows to a particular width and height instantly

I propose two solutions

wmctrl

Install the wmctrl package, then you can resize your window with

wmctrl -r string -e 0,left,up,width,height

where string is a substring of the window's title, (left,up) are the desired screen coordinates of the upper left window's corner, and (width,height) are the desired window's dimensions.

For a more precise window's choice, run wmctrl -l, which will give you a list of records containing window's ID, screen's number and window's title. Then you can resize a particular window by ID with

wmctrl -i -r ID -e 0,left,up,width,height

More information in wmctrl man page.

devilspie

Install the package devilspie, then create the folder ~/.devilspie and a file my-name.ds in this folder, with content:

(if (is (window_name) "My Window") (geometry "widthxheight+left+up"))

then execute devilspie to resize your window.