I have no idea where to park - anywhere, I guess, depending on trails we are biking. How long? Hard to say. 3-4 miles depending on terrain?
The two most popular places to park are the main lot near the boat launch and Ferber Circle on Cherry Ridge Rd. (off of Clinton Rd). Smoother stuff and doubletrack near the boat launch, technical singletrack and fireroad from Ferber Circle.
From Ferber Circle you could do Porqupine, then connector trail over to Banker and up to Plymouth. Moderately technical, nothing too crazy. A couple hike-a-bike sections for the kid, but pretty fun. That would be an out-and-back. Note that Plymouth has some bridges that are real slippery when wet. They're not high off the ground at all, but sketchy after a rain,
From the main lot you could pedal up the road to Iron Mountain trail to old Wawayanda Road, which links back to the main road in. 100% doubletrack. Good for a nice spin, I do it on my gravel bike.
Also from the main lot you could hit Wingdam to Sitting Bear to Laurel Pond back to the main lot area. There are some techy spots on Sitting Bear for sure, but it's a cool trail that rides right along the lake. The Laurel Pond trail headed back is 100% doubletrack.
This is a big park, tons of options.