Do giant size characters get plus 1 to their defense against ranged attacks if they are in hindering terrain?

i believe they do but i think they have to be fully in the hindering terrain.

Yes, they do. If the hindering terrain is between the figures, though, it can be ignored by drawing a line of fire to the figure with great size at a different elevation.

Clarification: When targeting a multi-base figure, remember that you can target any of it's squares that you can see, and also that squares of a multi-base do not block line of fire to other squares of that multi-base. This, combined with the "can attack them at multiple elevations" gives you quite a bit of lee-way in shooting at them.