I ride a 61w in the park so you're fine on a 59 especially being a taller and probably bigger guy.
A lot of people ride symmetrical duck for park but ride whatever is comfortable.
Learn how to ollie and get comfortable doing that before you try jumps. Then jump/ollie off rollers and side-hits before you take to small jumps. Bring your knees up when you jump and flex/bend when you land so that your legs absorb the landing rather than sending a shockwave up your spine.
If you are slipping out on boxes doing a 50/50 (you are riding straight down the box with your board pointed in the same direction as the box) then you definitely aren't balanced. When you get on the box you need to be very flat-based, you can be on edge because there's nothing to dig in to on the box.
On boxes try to look beyond the end of the feature, not down at your feet.