Start from how severe a wound you'd expect it to do on a Size 0 creature and work from there.
- I'd expect a Light to Medium wound.
- I'd expect a Medium to Heavy wound.
- I'd expect at least a Heavy wound, a kill is likely.
4,5) You're dead.
So, off the top of my head, I'd say 1) +5, 2) +10 to +15, 3) +20 to +25, 4 and 5) don't bother rolling damage.
An other approach would be to try to scale the target up to reduce everything to the first case. How big a giant do you need that a castle would look like a baseball? What's his damage increment?
One magnitude dedicated to a size increase multiplies the size by a factor of 10. The base individual size is in the guidelines for the specific form.