Put a marshmallow in your microwave oven. It will get large. Some of them will swell up to the size of a tennis ball. They become very chewy. Make sure it is on a plate rather than paper of any sort, since the part in contact with the surface will caramelize and stick. As for the time, well, you just have to watch them. When they stop growing, they are ready. It usually takes less than a minute. After they are removed from the oven, they will begin to shrink again, and the soft center will begin to harden. It is quite different than a marshmallow toasted over an open flame. Ah, the joys of technology!