If you have reserved an intermediate, technically that is all you are guaranteed. If you can't fit in an intermediate, you can always go to the counter and do an upgrade, however there is a downside to that. You are, at that point, at the mercy of the rental agent and they can charge you a little or a lot for the upgrade. You can negotiate, but if you HAVE to have a larger size, and they know it, they can make you pay.
All that being said, from what I have seen numerous times here on the DIS and in my own personal experiences, they do usually have full size cars on the aisle, which of course using Emerald aisle service you are welcome to take. If you are really nervous about it and are pretty certain you MUST have a fullsize you may want to go ahead and reserve it, but if you want to chance it, you will probably find one on the aisle for the intermediate price.
Good Luck.