I would say setup around two or three galaxies. One planet at 50, 150, 250, 350 then to the next galaxy the same. That is what i planed to do but it wasnt realistic cause most spots around those were taken. So i just tried to be at the MAXImun, the longest distance between my colonies is 200 systems. I realise that is long but my first six colonies are in galaxy 1 and 2. I have one planet in 3 and one planet in 4 just in case. The ones in 3 and 4 are just mining colonies, i figured since they are less densely populated they are less likely to be raided and I was right. Then get probed maybe once or twice a week...
EDIT: The moral of that long winded story, try to stay at maximum 100 systems between colonies. It gives you the best coverage of the galaxies.
Mine is only 8400 km in diameters Its average I guess.
Also, seems like such a big distance with everything in different galaxies, why not just take over and go nuts with one?
Lastly, any suggestions on getting those far away planets? Cause they always seem to be taken before I can get to them