The planets which get the highest field numbers, on average, are 4-6.
The next biggest, 7-9.
The 1-3 planets are sigificantly smaller.
(Although I got a 178 field #15 planet)
These are only average values... If you get less than 100 fields I'd say surrender the colony before spending too much time on it.
EDIT - Except 1-3 planets, then 70's good.
Planetsizes:
planetsize:
pos1: average: 64, 60% between 48 and 80 fields
pos2: average: 68, 60% between 53 and 83 fields
pos3: average: 73, 60% between 54 and 82 fields
pos4: average: 173, 60% between 108 and 238 fields
pos5: average: 167, 60% between 95 and 239 fields
pos6: average: 155, 60% between 82 and 228 fields
pos7: average: 144, 60% between 116 and 173 fields
pos8: average: 150, 60% between 123 and 177 fields
pos9: average: 159, 60% between 129 and 188 fields
pos10: average: 101, 60% between 79 and 122 fields
pos11: average: 98, 60% between 81 and 116 fields
pos12: average: 105, 60% between 85 and 129 fields
pos13: average: 110, 60% between 60 and 160 fields
pos14: average: 84, 60% between 42 and 126 fields
pos15: average: 101, 60% between 54 and 149 fields
The fieldnumbers are posibilitys, different numbers are always possible
From:
http://www.ogame224.de/forum/thread.php?threadid=590