Not only size, eg. duine mòr might mean a great man as in famous, one who did great things in his life, renown, chivalrous, mighty, etc. But it also can refer to size, great, large.
As for tha seo adj versus seo noun – yes, you’re correct. :)
Seo noun (eg. seo cat this is a cat) is a shorter way of saying is e seo noun (is e seo cat this is a cat) – with the copula which is used to link noun phrases; while tha seo adj is a usage of the bi verb used to describe noun phrases with adjectives and prepositional phrases. See also my comment under "Tha sin dona".
EDIT: I have written much longer and more comprehensive Guide to Gaelic to be, the substantive verb bi, tha & the copula is now. :)
Gaelic has two distinct to be verbs, you cannot mix and match them. You cannot say
seo mòr just as you are right you could not say tha seo tunnag – those would be both incorrect, grammatically nonsensical sentences. You cannot use nouns as the predicates to the verb tha, and you do use tha with adjectives.
I explained it in more depth in, among others:
- Seo vs Tha seo: https://forum.duolingo.com/comment/38564893
- "Tha sin dona.": https://forum.duolingo.com/comment/35388178?comment_id=35388600
- "It is another sandwich.": https://forum.duolingo.com/comment/38781757?comment_id=38782559
- the second part of my post in Tha ... ann https://forum.duolingo.com/comment/38688604?comment_id=38716029
Also, it is briefly explained in this very discussion already, in one of the posts above.