When you colonise a planet, you get a pseudo-random number of fields. The calculations for the fields is as follows:
Calculating Planet Size (number of fields)
- pos1 to 3: 80% between 40 and 70 fields (55+/-15)
- pos4 to 6: 80% between 120 and 310 fields (215+/-95)
- pos7 to 9: 80% between 105 and 195 fields (150+/-45)
- pos10 to 12: 80% between 75 and 125 fields (100+/-25)
- pos13 to 15: 80% between 60 and 190 fields (125+/-65)
The percentage is the chance you have of getting that range of fields. So for positions 1 to 3, there is an 80% chance you will have a colony with between 40 and 70 fields. The remaining 20% is the chance of more of less fields. It is possible to get 300+ fields on a colony. But you can get extremely low number of fields.
The more fields you have, the larger you can build on your planet. Each building levels takes up a certain amount of fields. As you increase your building levels your fields are used. Building defenses does not use up your fields.
Here is an example of how the field usage is calculated. This example is provided by a member of the PRGM Alliance
If I have a level 12 metal mine, level 8 crystal mine, level 7 deut. mine, level 12 solar plant, level 5 fusion plant, level 4 robotics facility, level 3 shipyard, and a level 4 research facility, I would have used 12+8+7+12+5+4+3+4=55 Fields. Even if I had 1,000 defensive structures (missle launchers, small lasers, ion cannons) my defense wouldn’t take up a field.
If you colonise a planet that has very few fields (less than 150) I would recommend using that planet as a farm. Build up your resources as high as you can, and put a level 1 Ship Yard on the planet. With the level 1 Ship Yard, you can station a Large Cargo Ship there (send it from one of your other planets). Now you can ferry resources to your other planets for research and fleet preparations.
If your planet is midsize (around 150 or so), you can build some simple defenses to keep your farm safe.
Just be sure save your resources frequently (or ship them to another planet). If things get destroyed, just rebuild them. In most cases, you won’t get attacked until you get higher up in score.
Note: The calculations used above are taken from the Help & Questions Thread at the oGame forum