Great question. Since pants/trousers are a plural noun, you would only use a plural verb to match it up. So, you cannot use 'it';you have to use 'they'
Your sentence: I bought new trousers. I tried THEM on once again at home and realized THEY didn't fit me.
However, if you are using a collective noun like "pair", you would use a SINGULAR VERB
Ex. My PAIR of trousers IS ripped.
My trousers ARE ripped.
Hope that helps!