3 series estate (E91) here – can’t comment on the saloon, but I believe the floorpans are identical.
Boot is pretty small, but the back seat area is pretty roomy (lots of legroom), so when the seats are folded it’s a decent size. The gap between the wheel arches isn’t the widest, but it’s wide enough. MTBs go in (lying flat)comfortably with the front wheel off. Roadies can go in complete.
The big downside is that the gap between the boot floor and the load cover is fairly small (RWD = high floor). One bike goes in fine, but if you want to lie 2 flat on top of each other, either you need to take the load cover out (it’s a big old lump), or you need to take the pedals off.
If you only need to move one bike around, the saloon may well be OK at that point, if you need to move 2, then estate all the way.
(TBH, I can’t understand why anybody would buy a 3-box saloon these days. Estates are SO much more practical, and generally look just as good.)