You never use "van/vannak" when saying what something is or how it is.
If you saying, "this is a clock," it's just "Ez óra." If you're saying "this building is small," that's "Ez az óra kicsi."
You only use "van/vannk" in the third person if it's modified by an adverb or question.
Hol van az óra?
Itt van az óra.
Does this help?
"The clock small". Kind of like "me Tarzan".
But it may help if you note that it is only in the third person that this happens:
I am small - Én kicsi vagyok.
You (singular) are small - Te kicsi vagy
He/She is small - Ő kicsi (implicit "is")
The clock is small - Az óra kicsi
We are small - Mi kicsik vagyunk
You (plural) are small - Ti kicsik vagytok
They are small - Ők kicsik (implicit "are")
And you can see that the verb varies with singular/plural and the person. So there is no need for the personal pronoun, it can be omitted.
And one more note: in the third person it is not optional to omit the verb. The word "kicsi" here in itself means "is small". Putting the verb "van" after it would render it meaningless. It would be something like "is small exists".
But in other cases you do use the verb even in the third person. In the above case we had an adjective (kicsi). Here, with an adverb, the situation is different:
I am well - (Én) jól vagyok.
You are well - (Te) jól vagy.
She/He is well - (Ő) jól VAN.
We are well - (Mi) jól vagyunk.
You are well - (Ti) jól vagytok.
They are well - (Ők) jól VANNAK.
So, from how I'm interpreting your question, you're basically asking about the difference between attributive adjectives (those that modify nouns directly, and which, in English, come directly before the noun) and predicative adjectives (those that are part of the predicate, and which, in English, come after a verb like 'to be' or 'to become'). Is that right? You also asked about the use of the verb 'van', but since that's already been answered, I'll focus on adjectives.
In English, the position tells you which kind of adjective is being used:
"This is a small clock" (attributive)
"This clock is small" (predicative)
In German (chosen as an example since it's your highest-level language), the position is also important, but the form of the adjective is usually also different:
"Das ist eine kleine Uhr" (attributive)
"Diese Uhr ist klein" (predicative)
In Hungarian, the position is also the main indicator, as attributive adjectives always come before the noun, but with this particular adjective, there are two different forms between the two uses:
"Ez egy kis óra" (attributive)
"Ez az óra kicsi" (predicative)
I hope that helped clear things up a bit.
I'd add a couple of things to that.
Kis and kicsi is sort of an irregular thing, "Ez egy kicsi óra" is also grammatical but somewhat rare. This must be one of the only examples where attributive and predicative adjectives look like different words. (Fun fact, the situation is a bit like with két and kettő...)
What's worth mentioning regarding predicative adjectives that they have to match the subject in number. So, Ezek az órák kicsik/nagyok/szépek