Planning is very helpful, especially if you can't seem to go past 10k words before getting bored. Have most of the thinking work done so that all you have to do is write it up, and fill in the gaps. Or write one shots. .

Of course there's more to it than that. I stopped writing my fic (a little over 20k) and already knew it was a flop but could not quite see. I doubt I would have worked out what went wrong without talking to other people to get another perspective.

So either make sure you have a beta reader (or ten) or get some feedback somewhere else on how your work is going.

No point in writing a 300k monster only to realize how bad it was and what a waste of time it was.

 

The way I see it, in fan fiction length matters.

Size matters a lot in a world of insecurity, so authors who write more write better in the end, and are thus enjoying a greater respect than authors who don't make the effort to be prodigious in their writing.

 

It has nothing to do with insecurity.

Length matters because if something is too short it doesn't convey enough detail. It becomes not a story but a summary. A story can be too long too: one of the primary characteristics of good style is concise writing. It get get too wordy or lack focus, both of which affect pace and can tire out the reader.

So yes, length is important. It is inseparable from style, both within a single sentence, and when looking at a chapter/story as a whole.