"The blouse is as big as the skirt."
Translation:A blusa é tão grande quanto a saia.
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.)