Why 1280 x 800 is not supported?

This is the standard resolution of all 13 inch (non retina) MacBook Pro's for example and many other Laptops before 2012 I think.

Wikipedia: "WXGA is also used to describe a resolution of 1280x800 pixels, with an aspect ratio of 16:10. This was once particularly popular for laptop screens, usually with a diagonal screen size of between 12 and 15 inches, as it provided a useful compromise between 4:3 XGA and 16:9 WXGA"

Or is it possible to use self defined resolutions?

Is possible, open the explorer (Tools/Explorer), go to Game, GameWindow Resolution, set your resolution. The resolution combo box will now show "---" btw. But I think that 16:10 isn't very wide spread and you might be better with 16:9.

Thanks!

PS: Some adventure gamers like 4:3 (= 16:12)
I think this should not be a technical decision. The aspect ratio should be a desing decision! That would be as if you were prescribing analog painters the aspect ratio of their paintings... I would also play a quadratic Adventure
That reminds me: http://gorogoa.com/

Sounds about right to me. My previous laptop had a resolution of 1280x800. This one has a resolution of 1366x768, but the intended resolution that you set your game to is not decided by the resolution of your laptop/monitor. You are making the game for other people no? Most people have 1080p monitors these days so it's best to use the standard HD resolutions @ 640x480 (480p), 1280x720 (720p) & 1920x1080 (1080p). The game I'm working on is 1080p, it pretty much scales down perfectly to my 1366x768 laptop screen (I mostly test on my laptop because I can't be bothered switching my secondary hd monitor over as my primary screen each time I want to test something, unless it's vital I need to see it in its actual resolution).

P.S: don't forget that you can always draw in the black bars yourself & add them into an interface.

I'm thinking about game distrubution and 4:3 and 16:10 monitors are a rarity. Tablets are the exception. Of course nobody says what resolution you have to use. And analog painters aren't painting digital, digital painting has always a technical dimension. You are free to do anything