No rules for posting really. Your story and be long, short, just poems, musings. There are no rules other than the content guidelines ( no glorifying self-harm or abuse etc). Give it a shot! You really have nothing to lose:-)
As for short stories? Im not good at them personally. After Humanity was meant to be a short story... it finished at 125k hahaha. But maybe you will find short stories work for you! Give it a try. Honestly writing has become such an integral part of my life now and i love encouraging others to try it for themselves
Wattpad is a wonderful welcoming and helpful community to try out work on i think. U can find people in the club forums too who might be able to help ( or mutually gripe about writing;-)). Im a community ambassador ( club mod) so if you want help getting started there feel free to pm me :-) Best of luck jumping into you writing!