How big does a driveway need to be?
The size of a standard parking spot depends on where you live. For example, in the US the width will usually be between 8 and 10 feet, and 18 to 20 feet long. But in a European country where the average vehicle is smaller, the average parking spot will also be smaller.

Something to consider when designing your parking area: space outside the "lines". You need to have room to open your door and get out without causing damage to the car or whatever is next to it. Make sure to account for that.

Additionally, you need to think about turning room. If the alley is narrow, you may not be able to turn in to your parking spot if the spot is too narrow, especially with another car in the next spot.