If it's your ascending aorta, particularly the aortic root, then it may decrease in size with ARB and/or ACEi therapy. Let me know if you're interested and I'll write more; otherwise, I won't waste my breath.
Unlikley. But your goal now is to prevent any further enlargement. This is best done by controlling your blood pressure aggressively, especially with medications called beta blockers and you should also consider being on medications called statins.