Large Taxi/Mini Van available at CDG Airport ?

You will have no trouble getting a van taxi for 5 at CDG. If your destination is in Paris (not clear what you mean by St. Denis area -- hope it is not the suburb of Saint Denis as it would be a poor choice with a family -- the street in Paris by that name is fine) then the cost will be 54 Euro -- 50 flat fee to right bank and 4 Euro for the extra person (the flat fee is for 4 and luggage). If it is not within Paris then it would be on the meter but this area is north and so probably would not even be that much -- meter cost plus 4 for the extra passenger. to return to the airport book a g7 cab -- they have an English speaking phone line well advertised. Make clear 5 and luggage and they will send a large cab.

Large taxis are available. A point worth remembering is that taxi drivers are not responsible for lifting and loading your luggage. Most will but it is your responsibility to load your luggage if it is heavy or bulky. Drivers have the right to refuse luggage which, in their estimation, may exceed their vehicles load capacity.

What area you mean specifically by St Denis area will affect both price, it you are not staying in Paris, and the overall safety of the neighborhood.

Yes, they are available. The person running the taxi line will make sure you get the taxi size you need.

Follow the "Taxi" signs in the airport all the way to the line. You may encounter people who are offering taxis before then. You have to go to the line, or you risk being charged much more than 54 euro. Even if the line looks long, it moves efficiently.

We used Super Shuttle last year from CDG to our hotel for our group of 8. They picked us up in a large van with plenty of room and the service was very good. I believe it was € 70.

Shuttles are notorious for unreliability in France and if you pre-pay and they don't show, you will be at fault and they won't give you your money back -- so if you do book a shuttle don't get one that requires pre-payment.