Emerald aisle is traditionally a full-size car or above. Highly, highly unlikely that the cars on the Aisle will be actual mid-sizes. You can check out the Cars at BOS thread to get an idea of what types of vehicles that station normally has available.

You can always ask station staff for a larger vehicle if what's out doesn't work for you. But there's no guarantee they'll give you one. So your best bet here would be to make a backup reservation for a larger vehicle (e.g., SUV) if that's what you truly need, and then cancel it from your phone if you find something that works on the EA for cheaper.

Keep in mind even most "mid-size" SUV's are rather tight on trunk space and the Uppababy is enormous. This is a recent Mercedes GLC300 rental I had with one large suitcase and 1 Uppababy stroller taking up the entire trunk.