So I'm looking to buy a low wattage amp head, the Orange Tiny Terror (7w/15w) in particular. I'm going to be using it for practicing at home, band practice and smaller gigs. My question is what wattage cabinet should I buy? I know a shop offering a great deal for the Tiny Terror and an 80w cab. Will the volume be manageable at bedroom levels with such a setup or do I have to look for a less powerful cab?
Thanks in advance.
You misunderstand, I think. Cabs are passive - the 'wattage' rating is, simply put, the most powerful amp they can handle without breaking. So any cab rated above 15W will be fine, maybe a 1x12 with a Greenback.

The only difference between cabs' volume is the speaker's 'sensitivity level' or SPL. For example, the Vintage 30 has an SPL of 100dB, which would make it sound a little louder than a Greenback, which has an SPL of 97dB. Either is still going to be a hell of a lot louder than 'bedroom volume'
The power rating needed is indeed at least 15W. But the speaker itself has a large effect on the sound. Ask the shop that has the deal to try different cabinets. The 80W cabinet may sound good or it might sound bad to you. Specifications are one thing, sound is another.

If you can, also try both open back and closed back cabinets. If done correctly, the sound is "tighter" with closed back - less peaking, but less volume with the same amplifier power.

There are cabinet designs that allow you to do both open and closed.
I believe that low wattage speakers sound best when they are played with an amplifier that is similar (but always lower) than the speaker cabinet. Which means that an Orange Tiny Terror through an eighty watt speaker won't sound as good through a 50 watt one. But you will have less headroom when playing live.
If you want to play live with the Orange, I'd suggest a 4x12, 60 watt (or something higher), Vintage 30, G12-75T or G12 Classic Lead speaker cabinet. That way, as sound moves in a medium, - in this case, air - the larger amount of speakers mean that there is more air physically being pushed forward (more sound)