TT says our family's max contribution limit to an HSA is $7900. But we started our high-ded HSA on May 1, 2018, so we only had it 8 months. Shouldn't it be prorated?

There is a rule called the Last-Month Rule that allows you to deduct the full yearly maximum amount to your HSA if you have HDHP coverage on December 1st of the tax year.

The catch is that you must maintain that coverage until the following December 1st.  

So, with that rule in place, you can contribute up to $6900 to your HSA for a family HDHP plan for 2018.