"My coat is too big for her."
Translation:Meu casaco é grande demais para ela.
Mmm ... perhaps that's less clear because that translates "My coat is so big for her" and seems to have lost the idea of "too big".
In Portuguese we say demais. "Demasiado" is frequently seen in Spanish. "demais" comes after the adjective. A general rule: for = para, by = por. But prepostion is something particular in every language, so there aren't prepositions translated always the same way from one language to another. It's something to dig deeper in order to get it right ;)