Most properties use the ADA Guidelines as their guideline for minimum space width. The guidelines state that spaces need to be a minimum of 8 feet wide. That’s a good number for a minimum. A better number is 9, but depending on the size of your property and the typical size of the vehicles who park there, you may have to make compromises.
Less than 8 feet means not even my Honda Accord can open its door without hitting the guy next to me. Most properties that choose to make spaces less than 8 feet wide think they’ll get more parking spaces. Sadly, its not true and the reverse often occurs. You’ll get more parking stripes laid on your property, but you won’t end up with any extra “space”.
In fact, you’ll probably end up with less. When people are forced to park in spaces narrower than 8 feet, they realize just how small those spaces are. They also realize just how close someone else is going to park to them. And they don’t like it. So they park over the line, effectively occupying two spaces instead of just the one. This type of parking spreads like the plague and pretty soon you have 20% of people on your lot parking over the line to save on dings on their cars. In addition to that, if someone has to squeeze between two cars and there is only exactly 8 feet to do it, they just aren’t going to fit unless they want to crawl out of their trunk. So the space becomes unusable. Once that happens, you’ve really reduced the number of spaces in which someone can realistically park.
If you go wider than 9.5 feet, either you have an amazingly huge parking lot where the number of spaces isn’t important or you have really cool hobbies. If you have a fleet of 18-Wheeler cabs or full size Hummers that you need to park, then I can see the need. Other than that, 9.5 feet is plenty generous.
One more note: Make sure whoever paints your lines actually gets out a tape measure and paints them of equal size. We have done the re-striping work on so many properties where the spaces will range in size from 7 feet to 15 feet (no joke!) wide. The disparity looks pretty ridiculous and creates confusion for people trying to park (especially if the lines are faded!).
What we generally recommend are 9 foot spaces, all the same size, distributed throughout the property. With that, people feel comfortable parking and all the spaces look uniform and welcoming.