It can be caused by having a DPI which is different from the standard. 96DPI (Or 100%) is the default. If you have it larger than the default the window will be created larger than you would expect it to be.
Go into your Original War directory and find OwarOGL.exe and OwarDD.exe. Right click on them and goto Properties. Click on Compatibility and then check the "Disable display scaling on high DPI settings". Click Ok. This should stop Windows from scaling Original War if your DPI is higher than the default.