We booked it around a month ago and changed the size of the box this week. I think you have to give the a weeks notice, but you can change from small to medium on the day. We did a mock up in our spare room but it was very close so thought we'd just pay the extra to avoid mega-stress on the day if things wont go in.
We are taking mainly small items in boxes (around 14 of them) and then some bigger stuff like guitars, amps etc. Loads of vacuum packed clothes! No furniture, No big telly or bikes or anything.
The boxing and itemising was a very long process for us so beware of that!