"The blouse is as big as the skirt."
Translation:A blusa é tão grande quanto a saia.
Can "quanto" also be replaced by "como" in this case? Or is quanto the main word used for these types of comparisons?
It's grammatically correct, but in Brazil you'll probably hear "tão - quanto" a lot more.
I don't think it's grammatically correct. As you said, it's a "tão - quanto" case, so if there is "tão", the "quanto" has to follow. It's, formally, a comparison rule.
Thanks for the clarification - I wrote 'tão - como' and was deemed wrong by Duolingo. However, the suggested correct answer was just 'é grande como', i.e. the system suggests I skip the 'tão' rather than exchanging 'como' with 'quanto'.
Can anyone give an example where 'como' would be used in a comparative sentence as opposed to 'quanto'?
You could use 'como', I would claim, but it gives a slightly different meaning (nuances). "A blusa é grande como a saia" would be more like "The blouse is big, like the skirt" vs. "as big as" expressed by 'tão - quanto'. (But then again, the generic disclaimer: I'm not native, neither in PT nor in EN.)
A blusa é quantos grande tão a saia was marked as incorrect... correct?