Does this refer to the physical size of his head or can it be used colloquially as in to ask if he is arrogant?
Physical, although I imagine most speakers would understand a dual meaning.
Does anyone have a list of all the genders and person forms of "Tha _ agam" "to have"
Bit odd to be asking that in a question that doesn't use the 'aig' form. Are you asking how to say 'I have, she/he has, we have, they have, you have etc?
So... "Is his head big" would be a different phrase?
Yes, it would be a different phrase. a bheil an ceann air mòr?