Can be as long or as short as you or the commissioning editor want it to be. There are those as short as 500 words and as long as a novella. As long as it's well written, no story is too long or short.
I would say between 2 and 4-6 thousand words is average, though.
It depends entirely on the individual publication.
You can go in your local library and look at the current Writer & Artists Year Book which is full of useful information like this.
A short story is between 1,000 and 2,000 words. most popular is 1,200-1,250. If a specific length is requested, say 1,250, then it must be exactly that length, it has a space to fill and must fill it, no more, no less.
For manuscripts less than 3,000 words, you must count every word and state on the front cover of the MS.
Daniela, that's a very stringent set of 'rules' there. I wonder who gave them to you, because they're by no means universal.
The truth is, there is no optimum length. It's the proverbial piece of string, I'm afraid. For instance, last year's Bridport winner's story was just short of 5000 words and that's not unusual. Then again, I know of several competitions where the word limit has been restricted to 2000-3000 words, and yet others that attach a limit as low as 500 words.
As I said, it depends on who wants the story and what the writer is writing it for. If it's just for you, then it can be as long or as short as you want it to be. If it's being written with a specific publication or competition in mind, then you would need to check the limits and rules set by the editors/organisers.
Any length of story submitted for publication is expected to include a word count - that goes for novels too.