Now, I know that they can't be hidden by other characters, outdoor blocking, or elevated, and a barrier indoors should wall off all sight to one, but ...

Can a barrier block all lines of sight to a giant figure outdoors?

Or does that fall under the 'outdoor blocking' catagory?

Outdoor blocking terrain and elevated terrain can block LOF to a giant character.

(It only mentions characters, not outdoor blocking terrain or elevated terrain.)

Depending where the character is, an outdoor barrier may block the LOF.

As an example, assume a grounded barrier:

LOF between grounded Giant Character and grounded standard character is blocked.

LOF between grounded Giant Character and elevated character is not blocked.

LOF between grounded Giant Character and soaring standard character is not blocked.

An Outdoor Barrier would be outdoor blocking terrain.

That cleared it up for me - thanks for answering so quick!