Something to remember, too, is that if the agent reads the whole thing, she WILL know how many standard pages it is equal to, no matter what is done to the font or margins.
I'm not an agent, but I've done some contest judging, and after a while, my brain knew exactly where the manuscript should have ended, and I'd start thinking (at least subconsciously), "shouldn't I be on the last page now?" And then if I compared the fonts and margins, etc. to the other submissions, it was obvious that some fudging had happened. And I was only reading half a dozen beginnings over a long weekend; it's got to be even more obvious -- like muscle memory -- to someone who could read that many every day.
The thing is -- one more page or five more pages or even a hundred more pages isn't going to improve your reception (especially if the recipient is irritated that you couldn't follow a simple request). In my experience, the strengths that appear in the first page of the submission are repeated throughout the submission, and the weaknesses in the first page are repeated throughout the rest of the manuscript. It's not like page one through eight are kinda' sorta' okay, and then all of a sudden page nine and ten are brilliant, and the agent just has to represent you. (And if it is like that, you're better off revising to make pages one through eight as brilliant as the last two.) No, what happens is the first page is good enough for the agent to keep reading, and the second page is good enough for the agent to keep reading, and so on, until the last page is good enough that the agent wants to see more. Doesn't really matter if that last page is number five or number twenty-five.
One of the best contest entries I ever judged was also the shortest. This was several years ago (and the author has since gone on to be published with a major publisher), and I can't recall the details, but I think the rules were "one scene, but no more than ten standard pages." I got everything from multiple scenes to what was probably 15 standard pages. The one that was brilliant, though, was one scene and only about five pages. The author's conviction shone through the whole thing -- she'd written a good scene, and she knew it, so there was no need to fudge anything.
Trust your writing (or revise until you do trust it), and then send exactly what the agent/editor asks for.