That looks like an oakleaf lettuce. Oakleaf lettuce and other leafy (non-head forming) types are cut-and-come-again, meaning you can harvest them whenever they are large enough to cut back. That looks quite ready for its first haircut!

Thank you so much!!

Eat now whatever leaves look good, save the center small leaves to get bigger. Eventually as the plant gets taller the plant will start to flower (bolt), and not be as good to eat.

I pick the leaves from the bottom up so as to keep getting more lettuce as it grows upwards.

It's hard to see from the picture but I get the impression that lettuce is raised in the centre with smaller leaves, implying that it's already starting to bolt. Is it beginning to produce a stalk in the middle? If so you need to pick that out and start eating ASAP rather than beginning from the bottom.