what's the lowest kind of arti/mining/research planets would you colonize? (size and richness)
and how much do your artifact/mining/research planets usually make? (say production with the size and richness)
i just want to compare and see what kind of planets should i be colonizing. thank you.

It's completely dependant upon your rank, but a general rule of thumb is to use VL Rich as your starting point or 0. Each level in richness is a +/-1, each level in size is a +/- 1 (maybe 2). However, if you are in the upper midranks to upper ranks, most likely the colonization/invasion cost will be so great that it's not worth your time to colonize anything less than multi mega rich planets.

alright thank you. im level 198 btw and i've been colonizing average planets with extremely rich in artifacts and also small 2x-3x rich artifact. tell me what you think about the planets im colonizing? and what's the average of your artifact production per planet?

You should be getting at least 150 units/hr from any planet (assuming +12s). If you cannot get that amount, then it is not worth colonizing

I look for 200 an hr in production out of my planets VL EXTRich planets or better in size or resource.

okay, seems like my arti planets are good then because there making 150's per planet with +10s