I’ll be using Bedroom 1 myself, which means it needs an extra-long bed (since I’m 2m tall and even King Size beds are “only” 2m long). While it will be no problem to install a bed of a different length, it’s a good idea to settle on the width of the bed early on – mainly so that the bedside tables and lights can be installed with the right spacing.
In doing some research into extra-long beds I’ve found that it’s perfectly possible to get a custom bed made to any size you like – for a suitably large fee. One problem with having a completely custom-sized bed is that you then need custom-sized bed linen to go with it. That means it’s better to go with a (more) standard size and the nearest thing to a standard size that is more than 2m long is “Emperor” which Wikipedia claims is 2.15m x 2.15m although some other places think that 2m x 2m qualifies as Emperor and call the larger size “Large Emperor” (e.g. Vi-Spring, as sold by John Lewis).