Can you give me an example in English with "too big" where it'd be translated as "muito grande"? I can give you one where "so big" is "tão grande": O Everest é tão grande que o alpinista levou algumas semanas para chegar ao topo. But it is true that "tão grande" is also "as big": Your building is as big as mine.
Uhm... I'm not sure... I guess we are walking the thin line of interpretation. In all these cases, if you stretch the "u": muuuuuuito grande (I'm guessing that is the intonation you are talking about), I'd translate it as "reeeeaaaally big". "Too big" I'd really translate as "grande demais". But as I said, we are in the subtle realm of interpretation and yours is probably as good as (or better than) mine. Thanks for the examples.
"muito grande" is more like "very big", but depending on the context it can be translated as "too big". Compare: Meu pai é muito grande (my dad is very big - too big doesn't make much sense). Essa calça é muito grande para você (these pants are too big for you - grande demais works too). There is no exact translation.