I've grown beets in pots before, which were great for the greens. But the root bulbs did not get very big. I didn't mind too much as baby beets are very tender, so I still had plenty of good eats. But this year, I want to try growing sizable beets in containers in the greenhouse, along with carrots.
I haven't been able to order any soil to be delivered, so I can't start using my new raised garden. In order to get some veggies started, I'm using some of the styrofoam seed flats I picked up last month (see Bargain Gardening Supplies post). Some lettuce and sunflower seeds were planted in single containers. I planted Bon Vivant lettuce mix, and Mammoth Russian sunflowers. For this type of mixed lettuce seed, it is best to plant in containers where you can control the soil. If you grow it in regular garden soil, it can be difficult to tell a lettuce leaf from a weed when there is a variety.

Containers should be OK for these two root veggies: I chose a carrot variety that is stubby (left) - Paris Market Atlas Carrots, and one of the round beet types (right) - Touchstone Gold Beets.