Mine 13 1/2 wide. No problem except one car further from the gate has to park slanted a bit. If another 1/2 foot (ie 14 feet) just perfect otherwise 14 1/2 should be ok unless you want to reserve park.

14 feet for main gate with swing gate. 2 feet pillar and balance 3 feet for another small gate for people going in and out without using the main gate that will look better. If you use up all the 20 feet (19 feet actually) it dont look nice. Go round and see others then you know.

Of course the bigger the easier. My house 22 feet and I have 13 1/2 feet gate, one other car park senget a bit. I think 14 1/2 will have no problem parking both cars straight with little bit of steering. If you want to go straight in then you need to open up all your 20 feet wide house as gate.

The bigger the gate the heavy it is and heavy duty auto gate needed.

Depend on the weight la bro. Stainles steel is heavy but rod iron is solid heavier but slim. Another aspect is the bearing, get stainless steel ones. Again you got to make sure it is gravity level/balance which depend on kong fu (skill) of installer. If out of balance more unnecessary force is needed and stress on bearing and soon bearing KO and follows by the auto gate arm/motor. These are the thing to look at not just how strong or powerful the arm. That also tell you why some got no problem yrs after yrs and some cant last a year. It is not jsut rain water get into motor

It also depend how wide the gate can swing. If only 90 degree, can but some difficulties. If both swing 110 degree no problem. This is called maneuverability. Also if the first car simply park over its "lot" space then the 2nd get difficult. That is the first car has to park properly and the 2nd can park simply.

Through experience 14 feet swing gate is good enough. 14 half if you like. Any thing bigger than that dont look good for 20 feet wide house. Gate proportionality too big make your house appear to be small, not grant. Try to balance between practicality vs looks. Also if your front driveway and the outside road is not level you may have little problem positioning your cars. Arm gate will take some space vs the traditional ones which install on the ground. You can forget about folding gate. If you really want an easy parking you can opt for sliding type. You can then take the whole 20 feet opening on either side at a time. Not necessary to have a level gate, you can make it a "m".