70 x 90 would be a great space for your chicken flock and garden. Gentle slope, especially if high side is on North, and low side is on South is fantastic. Gives good drainage. And a South slope warms up more and catches more light so you can plant earlier in the spring. Big issue with trees is that their roots can invade the garden and suck all the moisture and nutrients out of your soil Roots will travel farther underground than the drip line of the tree. Pine needles and cones not an issue for the chickens. Beware falling branches in storms if you park the chickens under the trees! All the leaves from your trees will give you a head start on mulch/compost for your chicken coop and your chicken run, and your garden.
If you go the tractor route, some folks make the tractor the same size as their raised garden beds so the tractor can easily be attached to the top of the bed (or sit inside it's footprint) or move between the beds. My garden is too crowded to allow a tractor in it during the growing season.
One consideration of having the garden and chicken run very close together is this: If the run is a dry and dusty affair, there will be airborne chicken dander and even feces, even in an outdoor environment. Therefore, while I would not have issue with any crops which grow above ground and need to be cooked growing immediately adjacent to the run (squash) I would not grow any crops that are intended to be eaten raw, or even crops that grow underground (salad greens, carrots) immediately adjacent to the run. Keeping a DL run will cut way down on the risk of air borne, or even soil run off contaminants being an issue.
