Both turnips and onions grow the top half out of the ground. You can judge the size just by looking. Turnips can be used at any size, but can become pithy if left too long. Usually about a month harvest window. Onions also can be used as green onions at any size, but for mature onions the tops will fall over and die. That is when to pick and dry them. If your onions have not bolted (bloomed) to this point, they probably will not. They are biennials that trigger to bloom with increasing daylight. Seldom a problem with waning daylight. Those that bolt are best used promptly as green onions.

so is it good that they may not flower? or does it not matter one way or the other? I didn't realize that you could eat the greens of any onion. thanks for the tip!

If you want onions, it is bad when they bolt. They put all thier energy into making seeds and forget about storing energy. I never heard it refered to as greens, but green onions ( scallions) are quite popular. The whole plant is used in salads ,stir frys etc. Lots of folks grow bunching onions which never bulb just for that purpose.