Calculating Fuse Size
A first-order approximation would be:

\${22.5\text{W} \over 0.7} \approx 32\text{W}\$

\${32\text{W} \over 12\text{V}} \approx 2.67\text{A}\$

So a 3A fuse should suffice.


  ... would likely not blow before it wreck electronics within the camera.


That's not actually what the fuse is for. The fuse breaks the circuit in case something is already wrong with the camera or converter in order to prevent more damage from occurring.

In all honesty you could probably get away with an even lower fuse value, but you would need to use a logging (or at least high-refresh-rate) ammeter in order to find out how much current is actually being used by the converter and camera during normal use.