At the end you still ask the coach, the peeps here recommend, but you have to use your own judgement, and its not hard, all you have to do is ask.
Spacetime day 1:
ST: hey I'm looking to do a trial/join
coach: sure blah blah blah
ST: so how does this work?
coach: here are gloves, you can buy it here or whatever, gym culture blah blah blah
ST: what weight for the gear?
coach: blah blah 16oz, 12oz, whatever
---boom---
first gym I was with it was 12oz, despite research of many peeps saying use 16 in spars and 12 in practice. Doesn't matter, the coach said 12 is fine all around, so I used 12.
New gym I'm at, he laid down the groundwork and said 16oz in sparring, 12oz for other. So that's what I'm doing.
Every glove they had in a pile of dump thrown in at the gym were 14 Oz. I will buy 10oz for bag work and if they want to spar, I use the 16 Oz:s.