If you want to go very light weight, you could go for a pedal with a headphone out and practice with headphones on.
I have a Rumble LT25, it has enough oomph for my small room (for me, at least), but I seldom use it right now. I often just plug into my Focusrite Solo and use my Yamaha speakers attached to it (with a bit of cab sim in Reaper added to it), or most recently, I used the headphone out on my new Boss GT-1000CORE.
I often practice late in the evening, so my neighbours would probably be disturbed even by the small Rumble 25. That is why I tend to go the headphone route nowadays (also, I tend to play with a pick. Headphones blend out the clicking of the pick on the string quite well :D)
The general rule for bass amps seems to be pretty much: Go as big as you can afford, though.
Probably the best thing would be if you could shoot out a Rumble 25 vs a Rumble 40 INSIDE the room you want to play in, just to see if the 25 is enough.