| My thinking is that you should stick to whatever kind of guideline they give you--if any at all. When they say one to two pages, take them at their word; go all the way to the end of the second page if you want. If they groan when they look at your app, they'll have only themselves to blame, and if they're competant at their job, it shouldn't hurt you.
If they don't give a limit, I'd say keep it to three pages. Middlebury was nice enough to give me free reign, and in return I made sure my essay was a respectfully brief two and a half.
As for font and type size, I don't know. I'm very sure that no admissions officer wants to read something written in some tiny, hard-to-read font like Edwardian fresh off their 1000th application. On the other hand, the use of a font that is easy-to-read yet unique may be one more way to differentiate yourself from the group. It might leap out at the reader and shout "Hey! I'm not afraid to be different, but I'm not going to be an ass about it!" If you pick a font that re-enforces your character as expressed in the essay, it might just work.
Then again, I could be completely wrong. It might just look stupid and un-professional. I wonder what others think?
Also, make sure your essay is at least 1.5, if not double spaced. Think about it: if you'd just read 50 or so different applications (let's say right before lunch break), each with three essays (Common App essay, extra information, supplemental), would you really want to have to give that ounce of extra effort it takes to differentiate single-spaced lines of text? I wouldn't. Double spacing is easier on the eyes, and anything that makes the admissions officer's job easier helps you.
If you want to squeeze in a little bit more (let's say your essay goes just barely onto a third page), feel free to manipulate type size and margins just a bit (say, size 11.9 to 11.5 font and an eighth of an inch off the top, bottom and sides). Nothing really noticable; just enough to fit it all in on two pages and therefore look good.
Good luck. Now I have to get back to work on my "Why Swarthmore" essay. That DVD they sent me in the mail really is a gift from Heaven... | 
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