This does work for me.
If using Unity on 13.10, you can achieve this with ccsm (CompizComfig Settings Manager).
You have to have it installed (following command should've already be done)
sudo apt-get install compizconfig-settings-manager compiz-plugins-extra
Activate module Window Rules under Window Management category.
Click the button Window Rules, you should see a tab pane with two tabs: Matches and Sizes.
Click on Sizes tab and add any app you want to specify its size with the New button.
Indicate the desired width and height and to which windows(s) you want the width and height to be always the same.
Do the same as above but with module Place Windows.
Then try to close then open one of the windows you specified.
I'd like to include some screenshots but I'm told that I need reputation 10 at least before I can add images to my posts.
Screenshot: http://i.imgur.com/Sl156Gf.png (ccsm with above modules buttons on a fresh French install of Ubuntu 13.10 - Ubuntu Saucy installed on 06-dec-2013) -- Even if texts may differ from French to English, icons on the screenshot are the same on every install of ccsm.