They do not grow as large as collards at all. They are actually quite different. Collards put up a strong central stalk and the leaves grow off the stalk something like branches from a tree. Turnips, on the other hand grow straight out from the root making a cluster of leafy stalks in somewhat of a fountain shape. Your greens look very good. You can harvest them now for sure. You can just cut the leaves off and the plant will make more, or you can pull up the entire plant root and all. They will be good until your weather starts hitting into the high eighties with regularity. You'll want to get them all in and eaten or frozen before then.

Those look good. Pick the biggest leaves and eat them. (I cut them off with scissors because I'm a klutz.) Like Donna said, they'll grow new leaves out of the center. Repeat every couple of days.

I let mine grow a day or two longer because they cook down so much. Yours are certainly OK to harvest though, and they look delicious.

SHOW OFF.:-)
I would pick the large leaves on all but one plant. Let that plant's leave get bigger, then cook the large leaves by them self & taste test them, then you will KNOW what size leaves you like best.
I had turnips all Winter & never had any as pretty as yours.
I plant more collards then I need, so I can pick them young,small & tender. If I have more then I can eat, I can them or give them to friends who do not garden as much as I.