luggage storage in Paris

the major train stations have left luggage services where your stuff is screened like at an airport and then you put it in lockers. Montparnasse has this as does Gare du Nord. If you are taking the RER B to CDG then Gare du Nord would be a sensible place to check luggage. Or look at stations near where you want to spend your time. I don't think St. Lazare has left luggage nor does Bercy, but Gare de Lyon, and Gare de l'est do in addition to Montparnasse and Nord.

I have similar question, can they store a large piece of luggage?

You may have to look at an app like Nannybag or Stasher.

last Christmas holiday, we (family of 3 adults) stored our bags at Gare Montparnasse, using 2 lockers for 6 pieces of luggage for a few hours while we toured the Marmottan Museum and top of the Arc de Triomphe.
Upon return, the taxis stage right next to the luggage storage facility. Costs about 60 Euros direct to our hotel near CDG.
Alternatively, you can catch a Bus out front of the Gare to the RER line at Port-Royal Station.

BUT, the ease of the taxi option was well worth it!

" I have similar question, can they store a large piece of luggage?"
What are the dimensions of this "large" piece of luggage? You can find the locker sizes here.

Thank you! I like the option of leaving them in the lockers at Montparnasse and then taking a taxi to CDG.

I haven't used the lockers at Montparnasse, but when I used the lockers in Lyon at their train station there were several sizes -- large lockers could take several bags and certainly a large one and there were medium and small sized ones as well. You use coins for the lockers so come prepared with lots of coins.