Sounds like they are ready to harvest! You can pull them and chop the roots, or chop them off at the base while still in the ground. They won't regrow so it doesn't really matter how you do it. Lettuce can be harvested at anytime before it bolts (goes to seed). My favorite lettuce to harvest is when they just start to form heads, and I will pick a few of these tender yet small plants and let the rest get a little bigger.