Learn Turkish: Bomboş ve dümdüz tarlada bir at duruyordu. - A horse was standing in the completely empty and perfectly flat field.

These are intensified adjectives formed by reduplication. You take the first syllable of the adjective (for boş, it's bo-; for düz, it's dü-), add one of four consonant letters (m, p, r, s), and attach the whole thing to the front of the original word. Boş (empty) becomes bomboş (completely empty), and düz (flat or straight) becomes dümdüz (perfectly flat).

Both bomboş and dümdüz use the letter "m" as the extra consonant. Is there a rule for choosing between m, p, r, and s?

Unfortunately, there is no strict rule to predict whether a word will take m, p, r, or s. You simply have to memorize which consonant goes with which adjective. However, p and s are the most common, making these two words good exceptions to commit to memory! Other common "m" examples are sımsıcak (boiling hot) and yemyeşil (bright green).

Doesn't durmak usually mean "to stop"? Why does it mean "was standing" here?

While durmak does mean "to stop," it is also the standard verb for "to stand" (as in, to be in a stationary, standing position or to be located somewhere). For a horse just being present and motionless in a field, duruyordu is exactly the right word to use.