currently, x, y coordinates must be chosen and a map scaling factor can be applied (usually just have to modify the x-y coordinates for any changes.
Suggestion is: use a drop down menu with names of possible spawn cities, use a list or dict in lua to translate the chosen city to a "standard" x,y coordinate and then make the changes as necessary if map scaling is not set to default.

actually, it looks like there is another mod that does just this:
https://mods.factorio.com/mod/CitiesOfEarth

I'm tracking this request here: https://github.com/TheOddler/FactorioWorld/issues/12

But yes, for now use CitiesOfEarth, as this isn't high priority for me right now, since someone else did it already. Unless maybe I could work together with the CitiesOfEarth guy, we'll see.