How do you choose a new server now ?
I used to go to WoWRealmPop, look for a server that had a decent Horde or Alliance population, and then roll there. That way I could find a sever that was not empty but did not have server queues or massive lag.
Does it still matter? I’d like to roll new on a new server. So far I either found the server massively full with huge lag in Stormwind and such or so empty that there was literally no one in Goldshire even trying to fight the Horseman.
How do I fins a sever that is busy enough to have Alliance or Horde people to do content with but not slammed full?
Thanks for any advice 
wowrealmpop has more or less been replaced by wowprogress.com and raider.io these days. Keep in mind populations are just estimates, but these two tools are used by a good portion of the playerbase so it’s as accurate as we know of without seeing blizz’s internal numbers.
Quick note on instance queues like the Horseman, dungeon finder, and battlegrounds. Those aren’t just pulled from the server that you play on anymore, but are pooled from whoever is queueing in the NA region. For the PvE content, if you’re queueing as a DPS, you can expect longer queue times for these sort of things as most of the population plays as DPS. Similarly, expect longer queues if you’re not playing at peak hours as well. 
Also, with cross realm zones, (CRZ), each most zones are ,shared across several realms. If a zone gets filled, a new instance of it is spawned for additional players from those realms to enter.