When I edit MapSetupDefs.xml I can raise nebula frequency by doing something like this (e. g. for abundant):
<Nebula>1.0</Nebula>
(bigger values seem to have no effect).
I then get a map with a lot of very small nebula, but I'd rather like a map with fewer, but bigger nebulae. Is that somehow possible? This question was posed 1,5 years backs already (see https://forums.galciv3.com/465754), but there was no answer ...