I used to have 1280x1024 as my screen resolution. Trying out 1400x1050 works, but I like the 1.25:1 ratio so I tried 1400x1120 and 1350x1080, but they don't work. Can I fix this or are there limitations to what xfree can do?

I'm not an expert on this topic, but I'm sure there are limitations as to what kind of screen resolutions you can have on any kind of system. The most common ones are obviously:

640x480
800x600
1024x768
1280x1024 (occasionally 1280x960)
1600x1200

It is generally a good idea to stay within these resolution limits. I have not seen resolution sizes that fall somewhere between those, like you mentioned, 1400x1120 and 1350x1080. That resolution size is probably not supported by your monitor even. There are probably work arounds to this to make it work, but as I mentioned earlier, I am not an expert on this topic. However, not using a resolution size out of the 5 or 6 common ones I listed above tend to make lots of problems. So, unless you direly need to have your 1400x1120 or 1350x1080, I'd switch to 1280x1024.

I asked a friend, who said it has everything to do with the graphics card and the monitor. Some work, some don't. That's it. I'll stick to 1400x1050, which I'm getting used to now. The ratio is ok and I can stick more windows next to each other