Best Answer: Most publishers look for "standard length," from first-time writers especially-for them it's just a ballpark figure to let them know you have some sense of pace (internally) and the novel market (externally). It usually amounts to 80,000-120,000 words.
At bottom, though, the better rule of thumb is "as many words as it takes to get your story told." If that number is significantly lower than 80,000, be aware that some folks will put in a class other than the novel.
YA tends to hit around 50-60 thousand, adult/epic fantasy at 100k+, your general fiction around 70-85 thousand. It really does depend. I suggest looking into the publishers of any books you have that are similar in story/the same genre and look into what those company's guidelines are for submissions. You'll run into companies that won't accept anything over 100k, ones that prefer you stay around 50 thousand, etc. Good luck. P.S. 250 words roughly translates to one novel page.
Stephen King said in the afterword to "Different Seasons" which was a collection of novellas, that, although there's no real definition, under 20,000 can usually be called a short story, over 40,000 a novel, and in between, a novella.
you can approximate about 300 words per page - - depending upon print size and if there is a lot of dialogue.. which would give you in the area of 90,000 words for a 300 page book.. and tell your mum never to pay an agent.. they work on commision and get paid only if they sell a work.. beware the con of an agent who wants upfront money etc.. best to you..