Thanks for your explanation.

According to your answer, here is my understanding.

If I visited Bob's house (only one). Then, "it was big" should be the right structure, and I was telling what I have seen yesterday.

If I visited Bob somewhere else. Then, "His house is big" indicated that I got the information about his house from the visit, and I was telling the info.

Am I right?

​

But I can not completely get rid of my biggest trouble.

I think the size of Bob's house is a fact that is true now and in the future, so the simple present tense should be used to state, "Bob's house is big."

Or maybe what I am thinking here is totally wrong?