Good to see the Brunton worked. Thanks for feedback. Btw - copper is MUCH better than brass, and I don't (any more) recommend using brass for the fin.
Now, the MSR canister. That gas mix should work OK in the cold if it still has enough propane in it. If it had been used upright a lot in the cold (as Mike M suggested) it might however been reduced to mainly isobutane, with little propane left. Isobutane boils at -12 C (10.5 F), so if you were running it at 7 F it was a bit cold. With only a little propane left in the canister, the pressure would have been rather low, and this could have meant the stove would not rev up much.
The first solution to this problem is to pour cold water (NOT hot) over the MSR canister. Fill up the concave base with cold water. This will put heat energy at >32 F into the canister, warming it up from 7 F. Yeah - sounds strange, but the physics works.
Letting the heat from the stove radiate onto the canister also helps. Do NOT let the canister get so hot you can't touch it - EVER! But 'luke-warm' is fine: they are rated to withstand 50 C (122 F) continuous.
There is one other thing to check, and that is that you had screwed the connector onto the canister enough. If you hadn't done this the Lindal valve in the canister might not have been opened enough. This sort of problem can happen when the rubber O-ring in the connector gets very cold. The problem disappears when you take the stove back inside and it warms up - which can make for some very puzzled people.
On the whole, I suspect that the problem was that the half-empty MSR canister had little propane left in it. This is a well-known problem. Start with a full canister and always use it inverted.
> does the size of the canister make a difference
Not really. It's the fuel mixture that matters.