Gargantuan and larger creatures

[Oakbreaker]

ok so this should be answered relatively quickly, I am DMing a home game and want to have my players fighting on the backs of creatures, similar to with vehicles...so a gargantuan creature takes 20ft of space but how do the dimensions work for the rest of their body...how much of their physical body is within that space? What's a dragon's wingspan?
how much of the body is wing, torso, tail? etc...would like these answers so I could go ahead and cut out the paper grid monsters for these encounters

[Magda Luckbender]

You will need to decide on these things for yourself. The rules are silent on these topics.

[Small Angry Golem]

I remember seeing a table that stated what exact sizes qualified something as a certain size category. I can't remember where though, and it might have been 3.X stuff.

I do remember that you had to be at least 8ft to be considered Large, but can't remember anymore than that.

[Zippomcfry]

Its not exactly what you are looking for, but probably the closest you will get

http://www.d20pfsrd.com/gamemastering/combat/space-reach-threatened-area-te mplates

[Sauce987654321]

That would be up to you. Gargantuan+ monsters are almost always bigger than their square space, though.