I've had my tomato plants in for about two months now and the fruit stays tiny, like the size of a marble. I water them every two days and they're in a spot with plenty of sun. What am I missing? Is it a nutrient deficiency or something else?
Did you plant them in regular garden soil, or did you add compost or fertilizer beforehand? After two months with such small fruits, the most likely issue is that they're lacking nutrients, especially nitrogen and potassium - tomatoes are pretty demanding. Try adding fertilizer (it can be homemade, made from kitchen scraps) around the base every two or three weeks, and if you have it, a balanced fertilizer dissolved in your watering water. Also check that there are no tiny pests on the leaves that you can't see with the naked eye, because that can slow growth too, though it's less likely if the fruit is already formed.
You have a classic nutrient problem, but it's not just a matter of lacking fertilizer. At two months in and with tomatoes small as marbles, nitrogen is important, sure, but what's really limiting fruit growth at this stage is a lack of phosphorus and potassium. Too much nitrogen can actually give you lots of foliage and few tomatoes. What you need right now is a balanced fertilizer or one made specifically for tomatoes, which has more potassium and less nitrogen than a general-purpose fertilizer.
Also, two months is plenty of time for you to have depleted the nutrients in your initial substrate if you planted straight in soil without any prior preparation. What kind of soil did you use? If it was garden soil you just grabbed from somewhere without any amendment, that explains a lot. I'd recommend you add some organic fertilizer right now (well-made compost, worm castings, something like that) around your plants, about ten centimeters away from the stem so you don't burn the roots. Mix it in a bit with the surface soil and water thoroughly.
Watering every two days is fine depending on your climate, but make sure the water drains properly. If the soil gets waterlogged or too compacted, the roots can't absorb nutrients properly even if they're there. Sometimes the problem isn't that food is lacking, but that the plant can't get to it because of the substrate. Try the fertilizer thing and in two or three weeks you should see improvement in the size of your new tomatoes.
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