How are you measuring room inside? Using Golf balls, the interiors are withn a couple of percent of the same size--call it a draw. Using adult humans the 355 is way bigger.
Interesting question never occurred to me but based on responses, Not challenging just looking to understand this one too, good to know. Same wheel base, same doors, firewall locations the same as far as I can tell, same basic car. Bigger how? More leg room? Seats lower maybe so more head room? Any sense of where the extra room comes from? Is it actual room or perceived room? saw this for example: https://www.ferrarichat.com/forum/threads/interior-room-348-vs-355.195096/
They are exactly the same inside, same amount of room. Only difference is the 348 has a dash to floor center console (which I like) and the 355 does not (which I never liked). That can give the illusion of more room, and I suppose could result in more knee room for some drivers. But otherwise they have identical room.
I own both and they are within whiskers of having the same internal volume. With the top down, a spider wins by a large mile.
With all do respect Mitchell 4 of you could likely fit comfortably. Or you and 3 victoria secret models ha ha. Not sure if you could tell, but maybe. Especially since your one of the few on here that I know that has both.