Currently my letter of intent is 1 full page & 3/4 of a second page (including all the adresses and my signature). I've heard 1 page is standard but is it ok to leave it a little longer? I don't want to be the only applicant with a ridiculously long letter but it flows nicely and I really don't know what to cut out.
That's the biggest mistake of budding writers - getting too attached to what they have written.
It goes away with experience. Or it doesn't.
As far as having almost two pages... it IS too long. Most people do not have the patience or the interest to read two pages closely. Keep things short, sweet and to the point. You think people are going to be willing to spend time on trying to figure out what you actually tried to say, when there will be a lot of people telling them everything up front in an easy to digest form?
Now, WHY is it so long?
Have you written a whole paragraph long love note about why the program you are applying to is the best thing since sliced bread? Cut it down to one or two key ideas.
Have you re-told half you CV in the letter? Cut it all out, leaving only one or two key highlights (be they qualities or experiences - up to you, depends on the program you are applying to).
Have you gotten boggled with detail? Try to distance yourself from your writing and think - is that something I would need to know if I were deciding which of these N people to invite for an interview? Is this information going to help me make my decision? Details can take attention away from what's really important and hide in the overall mess.
Take the red pen out and strike everything that is redundant, unnecessary, or "just sounds nice" without adding to the substance.
If after doing the above four steps still hasn't gotten you to the one-page single-space limit... well, repeat until they do. Just about the only allowable trick, if you have just over a page - say, three or four lines that wouldn't fit - then make the paragraphs 1.5-spaced and take the full two pages with extra spacing. That may work, but obviously it's the means of the last resort.
Feel free to PM me for a more personalized advice.