Hey Beachgal,
In the U.S., the maximum length for a leash in a public park tends to be 6 feet long. It also gives your dog a little room to roam and sniff around while still keeping them mostly controlled. For this reason, this is the most common length you will find.
However, if your dog is hard to control, you will likely want the shorter leash. For example, I prefer using a 4-foot-long leash with our stronger, harder-to-control pitbull mix. The shorter length also keeps them closer to you, which can make them feel more secure.
If you have a retractable leash that is often significantly longer than 6', you can use a Sharpie to mark off 6' on the cord to know the length you should use in parks with this maximum leash length requirement.
I hope this information is helpful and you are able to find the right leash for your pup.