Long roots usually means you have let the water level go down. That looks mighty crowded (in the future) I hope they are all really small things?
If you trim roots, that means you curtail plant growth for a while. It will need to re-grow the roots before it gets any bigger.
Next time you could extend the grow for a leafy lettuce plant by harvesting just the outer leaves a little as a time as needed and let the inner leaves grow some more, then harvest the outer mature leaves again and so on. This is commonly referred to - cut and come again.