PS, the change in size between the two measurements is likely due to measurement error, which is common. There are a lot of places in the process for error to creep in, especially if both were done by echo. But, again, even if the root is "only" 5.5cm, that is way too large. I would consider the second measurement of 5.5 to be merely a confirmation that an aortic aneurysm is present and NOT a sign that the root is getting smaller.
Aortic root aneurysms do not fix themselves. They have to be repaired surgically. I would urge you to get a second surgical opinion, because the size of your aortic root -- even if it is "only" 5.5 -- is in a range in which there is a significant risk of dissection. An aortic root dissection usually gives no warning and is almost always fatal.
I looked at your profile and saw that you live near Chicago IL. That is lucky for you, because you need an expert surgeon and team, and there are three medical centers in Chicago that are nationally renowned for their thoracic aortic surgery programs. In no particular order, they are Rush, Northwestern, and U. Chicago. I would go, as quickly as possible, to one of those three centers for an evaluation.
Just thought I should add a couple details - I am 55, female and have some Marfan's symptoms but not all. I was 6' 1" at my tallest, but come from a tall family.