If the bag is the maximum width (45cm), it will depend on the depth and shape of the bag as to whether it fits. My 40cm wide bag only just fits because it is quite square, but a more curved shape bag that is 45cm wide (eg Rimowa) would probably fit.

As mentioned above, the crew will store it in the wardrobe if it doesn't fit. I've never needed this, but have seen them do it many times for others on the UD, including twice in the last couple of weeks.