@brmbrmcar: Don't be so hard on yourself. Your first two levels that you uploaded may have had some flaws, but that doesn't mean you are no good as a level designer. If you keep on trying, eventually you will understand the Supertux style, and you can make some really good levels. Even some of the devs here have created levels that received lots of criticism, but they kept trying anyways, and improved their level developing skills.
Here is a little hint for your level design. A gimmick or difficulty may challenge the players, but the aesthetic of the level will make them want to play a level again and again. In my personal experience, I would rather play an easy level that looks really good than a hard level that has no decoration or looks bad. As far as level length goes, I typically don't care how long a level is, as long as I'm having fun playing it, and as long as there are enough checkpoints. Try looking at other video games like Mario to get some inspiration for level design. With practice, you could be a great level designer.