Just a small point but what happens if you don't get a free upgrade and end up with the medium size that you booked? It's always pretty unwise to rely on a free upgrade at the rental desk as sometimes free upgrades don't happen.

If you've booked an all-inclusive rental (for example through this site), then if you have to pay to upgrade at the rental desk it will cost you far more than the price of a higher grade vehicle was if you chose to book this initially.

Trunk size isn't too huge on either a medium or full size car - there's a guideline on the website at the top of each forum page. If you have the 'hard' type large suitcases, they're unlikely to fit into the trunk of either size car (in my experience as the trunks aren't very high) but the stroller and one large hard type suitcase will probably be fine. If you've got the 'squashy' type suitcases, they'll probably fit fine in either.