Yeah, definitely ask...
I know two professors of mine, each of whom I've had three times, and thus know pretty well--
One of them always asks for a shorter paper...but in practice, the 20-page beasts that I write for him almost always do better than when I stick to the shorter suggested limit he prefers, because he's notorious for asking and writing "How?" at every single statement you make. You have to leave as little room open for a "How?" to pop up in the paper, and for me, that means that each and every thing I argue is defended, sourced, that the sources have sources, that the arguments have arguments supporting those sub-points' validity themselves as well as why they're included at all, why I referenced someone, etc.
So, while he prefers a 6-8 page paper, I've almost always gotten better grades with him by writing 16-20+ page ones, just because I know what he wants vs. what he says he wants.
On the other hand, I have ANOTHER professor who wants 4-6 or 5-7 pages.
Period.
End of sentence.
4 FULL pages, or 7 pages MAX.
If it's 7 page and one line on an 8th page, there WILL be wrath, because she specified 7, damn it, and you're expected to follow instructions!
Both are awesome profs, but operate differently, so know thyself...and know thy prof. ;)