Zmag,
Either of the fan sizes you have are fine for those sizes of rooms. The advantage of a 52" fan is that it will push more air. The disadvantage is that over time the blades tend to slope downward. The advantages of the 42" fan are the blades tend to stay straighter over a longer period of time.
Do not flush mount your fans if you can help it, you won't get any real air flow. Even the small 2" downrod is worth using if it will work for you. Some fans have this option.