I have been hoping to do the same but our Roomster has been ‘remodelled’ (not our fault).
There is a disability/wheelchair site that gives internal dimensions if you Google it.
From memory of my own measurings I think I’m right in saying it’s 120 cm to the edge of the boot floor/start of rear footwell (seats removed). I’m not sure about the length to the front seats as far forward as possible, but, the front passenger seat can be cranked back pretty flat to butt up to the boot floor. We have transported IKEA sofa beds that way.
That was going to be where he-who-is-taller-than-me was going to sleep, with me (5’2”) on the driver’s side, with the seat fully forward.
All the best