basically it depends on where you're going to sell it but for most places no more than 10,000 words.
Sme have even stricter limits, and will say that 5,000 words are the max. It again depends. If you go for magazines, 2,500 is perfect.
A more literary magazine might allow for longer. When in doubt check the publications.
For my own personal rule is no more than say about 3,000
Hope this will clear things up