Lettuce in general has a modest root system and will grow as big as the space will allow. That is why they do so well in smaller containers on our deck or patio. If the roots are kept cool, they will thrive to a size consistent with their environment. A half-barrel in a corner that receives some sun and a nice breeze makes a great habitat for lettuce. You can also under plant lettuce seed so that the little seedlings are just coming along when you pull up the larger plants.
We are planning to make a lot of salad and lettuce bowls for sale at the early spring farmers' markets. So today I started about 1600 seeds of many varieties. They will emerge quickly and be transplanted into single pots, six packs and a variety of bowls. The cool weather is fine with them as soon as they have germinated and - as an added bonus - their mortal enemies (slugs, snails, aphids) are dormant this time of year.
If you haven't grown lettuce before, give it a try. The plants are hardy and the leaves are tasty. They transplant easily and will thrive in a limited space. They come in a wide assortment of colors and shapes and make a gorgeous display - almost too good to eat. But eat them, please - they will grow back.