Ooops...have just submitted an abstract for a conference absolutely convinced that it had to be 250 words long (I just kept within the limit). Now I see it actually says a maximum of 500 words - do you think it's a problem? Should I ask if I can add something or just let it be? And why didn't I read the instructions properly BEFORE submitting?! :$

if you think 250 words describes what you have to say well enough, just leave it. You don't have to reach the max word count. But if you are worried that you could do a better job with more words, just ask them if you can re-submit an amended version (without saying anything about the word-count).