One of the main guidelines for online submissions used to be "keep it short." For early zines the guidelines usually asked for no more than 2000-2500 words. That compared to print publications where word limits could go as high as 7000-10,000 words.
As people have become accustomed to reading online and more print publications have made the move to online publishing, those word restrictions have loosened quite a bit. In the past few years word length for strictly online publications have increased, and, from what I'm seeing, 4000-5000 word limits seem to be more of the norm, though many use the same longer guidelines as print and some have no specific restrictions at all.
It is my opinion, frankly, that new writers, whether submitting for print or online, do best to keep their work to 4000 words or less. Works that go beyond 4000 words are usually either trying to do too much within the context of the story, have too many characters, or simply contain too many words. That is, the writer is using two or three words when one more effective word could work better. New writers do best to impose that discipline on themselves.
When it comes to publishing online, I think even experienced writers should think carefully before submitting pieces over 5000 words. I do much of my reading online, both in the form of submissions and published work. While I can read much longer pieces in print, for some reason online my mind starts to wander or I become, quite simply, bored. I don't think I'm alone in this. Perhaps someday a nueroscientist or neuropsychologist will research it (or maybe some already have.)
Suffice it to say it takes something pretty compelling and/or very well executed to hold my attention for more than 5000 word. So my advice to anyone submitting work for online publication remains, "keep it short" even if not quite so short as before.