Milan Metro Parking

I'm not exactly clear why you would rent a car to go from Malpensa to Venice. A train would be much easier and hassle free, especially because you intend to stop in Milan for a few hours.
You could take the Malpensa Express train from inside the airport and get to Milan. There are two Malpensa express trains from the airport, one goes to Milan Centrale station (52 min) and on to Cadorna station (36 min). The train to Venice departs from Centrale.
http://www.malpensaexpress.it/en/

You could take the Malpensa express to Centrale, put your luggage at the luggage store for a few hours, and visit the city for a few hours. When you are done, you go back to the station, pick up your luggage and board a train to Venice.
Trenitalia has numerous trains from Milano Centrale to Venezia (Venice) Santa Lucia station (journey duration: 2h 35min).
http://www.trenitalia.com/

The train is faster (you can't drive from Milan to Venice is 2h and 35min) and more comfortable. Unless there are 3 or more of you traveling together, the train is also cheaper. In Venice a rental car would be a total waste of money. Rental costs in Italy, including mandatory insurance, amount to approximately €50 to 60 per day (for a small compact), in addition the car cannot be used in Venice because cars don't float.
You would need to park it the whole time you are in Venice at one of the parking structures on Piazzale Roma or Tronchetto island for a stiff €25 a day.
If you intend to rent a car to visit areas in Italy where a car is useful (like the Dolomites or Tuscany's countryside), you can do so after you leave Venice.

Cascina Gobba has a major park-and-ride facility. It is also easily reachable from the A51 ring highway.

See it on Google Maps here.

Thank you Roberto and Andre.
The reason I am renting a car is when we leave Venice, we will be traveling on to Varenna, Pontresina, and then the Berner Oberland. I did not know about the luggage store at Centrale. I will consider taking the train as you suggest until we leave Venice and then rent the car. Thanks

I used the luggage storage at Milano Centrale last October, when I arrived mid-day by train from Lucca/Firenze, dropped off my bags, toured Milano for the afternoon, then picked up my bags and continued on to MXP for the night and early morning flight back to the States. For about 6 hours of storage, I think I paid about 5.5 euros per bag for the storage. You do need to allow some time for waiting in line for the storage, both to drop off and then again to pick up.

For sure you should not rent a car before you go to Venice. You should rent it, at the earliest, when you leave Venice. All rental car companies in Venice are located at Piazzale Roma Car Parking Structure. If you need a car, you should rent from Venice Piazzale Roma starting on the date of your departure from Venice.
If you are going to Switzerland, you need to be aware that if you rent a car in Italy and return it in Switzerland, there will be substantial international drop off fees. If you return to Milan to fly back home, then there will be no problem since there are generally no (or low) drop off fees if you return the rental car in the same country, even if at a different location.
Regardless of where you rent or return, you need to notify the rental company that you are going to take the car across the international border and that is stated in the small print of the rental agreement.
Be also aware of the Swiss Toll Vignette you must purchase to travel on Swiss freeways.