If space is really tight, then there will be a compromise or two somewhere, so I think it depends on what the main objective of the improvement is as to where you'll accept the compromises. Are you looking for a good shower as you only have an over the bath one at the moment? Is the toilet just so there is one downstairs, but you'll probably be dropping the kids off upstairs? How narrow is the narrow space you've got, and how long? Can the door open outwards?
If the shower is the priority, you can probably go as narrow as 700mm (by 1000mm or even 1200mm if you've got the length). Normally I'd always aim for at least 750mm width for the loo, but if it's mostly a wee only zone then you can get by with 700mm. Also, if it really is just that, ie no morning shaves or teeth brushing down there, then the combined loo/basin aren't that bad if you are only washing you hands afterwards after a quick splash, especially if there is a normal basin in an upstairs bathroom.
If, however, it's to be a full on second bathroom, then you definitely need a separate basin, so you'd probably need to go 800mm wide (does mean that an 800mm square shower tray is an option, although a bit of a compromise imo as I always look to get at least to 1000mm along one side, but again, as I said, it's about priorities and compromise). That way you can have a door kinda in the middle of the long wall, shower one side, toilet the other, and a 200mm deep basin on the wall directly opposite the door.