I'm no expert but I was told that anything more than 14 feet ( 7 + 7) is not advisable for swing gate. Also depends on the gate material wrought metal / aluminum. Mine's powder coated aluminum and 13 feet wide. I use a simple and fuss free CEI motor because I dislike motors above ground installation.

Anyway depending on the length of the driveway, using narrower gate will require you to negotiate a little when you enter the gate and try to park 2 C or D segment type cars side by side. I find with a 14 feet wide swing gate, each car is able to go in and park at the designated side by side space in the garage comfortably.

Are you referring to the size of the car park itself? What is the size of the house compound? Yes, you need to consider what weikee mentioned about blocking water meter and in some cases the electric meter if it's outside at the main pillar. Especially that you mentioned that your car park is 20 feet wide. weikee also mentioned about the esthetics of the house... Big gate with small front does look weird in terms of proportion. So you need to consider that.