how to get more fields?

Terraformer gives an extra 4 planetary fields, only use 3 as you will need the last one for another Terraformer. Lunar Base works the same way as Terraformer, only it's for moons.

You should never keep a small planet anyway, ditch it as soon as you get it if it's small. Aim for 250+ default fields.

They give you 5 extra fields; Lunar Bases are the ones that only give you four. But Istal is right, keep one empty slot open so you can get another Terraformer.