Marymoore is tough to beat for size, and it's really nice, but I'll put in a vote for the off leash area at Luther Burbank on Mercer Island. It's less busy, plenty of water for dogs to play in and the views are great!
Been out to Marymoor a couple of times and it seems like a decent dog park, being on the West Sound we usually go to Raab Park in Poulsbo and have yet to check out the new one in Silverdale, but will do so soon.
Would the Marymoor dog park be good a small dog? Our dog is a little shy around big dogs, and some parks have separate sections for large and small dogs.
drive to mill creek 15, 20 min north of seattle go to the park on the corner of 164th and the bothel everet hmy is huge and beautiful hundreds of trail alot of people take their dogs there they even have a playing field
Marymoore is really the biggest. Otherwise there are plenty of regular hiking trails in the area and people generally don't mind a well mannered dog offleash.