If you don`t like to spend too many turns searching for planets that are 10 size bigger than what you need and you have some free time during the dig hours , use gordo to pump them.
Personally keep 850+ wen non-paid and 950 when paid
im a collective on fast and have about 125 planets, so cant explore and everyone in my pr range isnt a newbie so doesnt keep large barrens on their borders and i am keeping planets with more than about 250 land