You can't.
This is a three year old bug with no solution.
You either have to use a different theme or do as I have done and give up trying and use keys to resize windows.
Mini Howto:
Alt + F8 to start resizing.
Use arrow keys to resize.
Pressing Return will save the resize, escape will revert you back.
The bug report is here and has hundreds of comments on it and thirteen duplicate bug reports on launchpad alone.
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.
Best Answer
As others have mentioned, it depends on the theme. Try a few out. What I frequently do instead is hold down Alt and middle-click (right-click in some environments) and drag to resize, which works anywhere on the window.
Note that many themes that seem to have a 1px border (or no border at all) actually do tend to have a generous invisible border just outside the window. You usually don't have to pinpoint a tiny window border to resize.