As a fellow writer I’d say to just write and practice as much as you can even if you never publish it! As for length I’d say whatever it takes to get to “the thing” as Bukowski would say. Some people need 2k or more words while some need much less. It’s all about finding your own style of writing, the people who want to read your work will show up regardless. In my opinion, as a reader I like at least 1k words but I have read really short fics too. Just find what suits you and get good at it! Sometimes I read really long fics, and when I’m done it felt short because it was that good. Just do what feels right. You say you’re new to writing fanfiction, but if you’re new to writing for enjoyment in general my suggestion would be to try to make the goal of setting up the first chapter solidly if it’s going to be a long one(multiple chapters).
YOU SAID IN YOUR TAGS TO JUMP IN WITH THOUGHTS SO HI, HERE I AM.
One of the things I love about fanfiction is that it’s the fucking Wild West and there’s so much out there and it’s so subjective. Everyone has their own standard on lengths and such and I guarantee you that no matter what the length is that someone will want it. Like yeah we all wanna read a nice novel length fic that’ll keep us occupied for a week and rip our hearts out, but there’s a huge value in finding tiny fics that you can read real quick while you’re on your lunch break or waiting in a line somewhere. I promise there is an application for all lengths of fics. 😀
I did wanna say though, as a writer, I find it super fucking distracting to worry about how long my fic is while I’m writing it. I use TextEdit to write a lot cause it’s really plain and doesn’t have a wordcounter and doesn’t show you page breaks, so you can like smash away and not have to worry about it. I try to think of my stories as having a beginning, a middle, and an end, and it takes as long as it takes to get there. Don’t let the number bug you. If you’re telling a good story it truly doesn’t matter.