The 1A cabins are a bit smaller than the other standard cabins - I think around 170 sq ft versus 185. Not a big deal. With only one bed on the deck the usable space or "walking around" room is essentially the same. Certainly a good choice if you are going alone. If I ever go on Carnival by myself I plan to go with a 1A if the ship has them.
From various reports I've read over the years, it seems Carnival will often move people out of a 1A to a 4A if the ship isn't full.