Why not 'the tall house'?
I believe that would be 'a magas ház'
Can't it be translated as "The house is big"?
No, big means nagy.
Can we also use magas in that way to say a child grew up? A gyerek magas. ? Like now he is tall
Yes. A gyerek magas. = The child is tall.
Would you actually say in english that a house is tall instead big?
Yes, you could.