I have a7'6" 3wt and a 9' 5wt. They both have their place and their pros and cons. The short rod is nice when there's a lot of stuff overhead and it's just a lot of fun to fish. The longer one is better for high sticking and for keeping your fly line out of the water when there are a lot of conflicting currents pulling your line in 15 different directions at the same time.
I like them both for different reasons.
6wt may be a little more than you need in the park for the size of fish and bugs you're likely after. Of course, you really need to think of where you'll be using the rod the most and what you're after for most of your fishing. If it is only for the park, I'd probably go with a 3 or 4 wt. I use a 5wt in most of my fishing for bass and bluegill and it is also fine for the couple times a year I make it to the park to fish. I also like the 3wt on some of the small streams around here to fish for sunfish and bluegill. It is also fine in the park.
I hope this helps.