To set the actual resolution for the game:
1. Select your Window Layout in the top pane
2. Select "Regions" in the bottom left pane
3. Right click the preview area in the bottom right pane and select New Region
4. Place it at the upper left corner of the screen
5. Set the size of this region to the desired resolution -- in this case 1920x1200
6. In the bottom right pane click on the Swap Groups tab
7. In Swap Group 1 (selected by default) change the 'Reset' region option to be this new Region. This is what ISBoxer will try to use for the resolution before moving each window into position. If you had 5 regions and now added 6, the region you sized should say 6 and would be Region 6. Select that here.
8. Close out the game if it's running, and Export. If you're playing WoW or a few other games, ISBoxer will offer to fix the resolution stored in the virtualized config files for you. You might have to export specifically on that PC for it to pop up the box.

If you're on Windows Vista or 7 with Aero enabled, you can use the Full Screen Stacked layout for the second PC instead (this would put the res of the game at the same res as the monitor by default), and use Video Feeds to show the other windows or views from them. You can also add a blank window to do this in: http://isboxer.com/forum/viewtopic.php?f=35&t=1222