Three very excellent choices, visiting will be key on this since they have a lot of overlapping similarities.
Cornell is definitely big, and the town is shared with Ithaca College so definitely has a lot of college type activities off of campus.
Dartmouth is small, but twice the size of Williams, and if you think Dartmouth is isolated, Williams is even more in its own pocket of the world.
On the East Coast, all three have great brand recognition, but outside of the Northeast, Cornell and Dartmouth are going to be better recognized when it is mentioned to general people.
Visit all three, but Dartmouth is the middle ground of these. It gives you the Ivy League education, resources, and job/intern opportunities of Cornell with the small-town rural community feel of Williams