A small productive city can really help the empire.
What helps the empire? Gold, science and maybe culture.
What do you need for the above? Commercial district, campus, and an industrial district to build it all. This takes a size 7 city. Swap out the campus for a harbor if you want more gold and trade routes, for an entertainment district for zoos to help nearby cities, or a theatre district to help house art or trinkets. Dispersed science does fine as there are no multipliers, you get the science from the buildings. More culture slots to fill are better. It's only the industrial powerhouse city that will build the space port and all the missions that needs to be super productive.
If you can drop in a few of these borg cities around your optimally placed on the terrain big signature cities, your empire will prosper. You can even add them in late as you can reroute one or two trade routes to your industrial cities and get them online quickly.
I'd set the first few cities wherever the terrain was best, all things being equal I'd keep them a little closer than ciV. Then fill in the gaps, settle any luxuries and strategics even in tundra. After environmentalism you can plant trees and lumbermills in tundra river tiles and make for really nice gulag camps.