Most of us would be a size 24mm. But yea sure, it's definitely okay to wait till delivery and ask the lactation consultant to measure the size for you before purchasing the parts, if you don't want to measure yourself based on the online guides.
Same here. I heard from my frens and relative that they only latch in the beginning.. so i didnt bother to check yet. Even that time i check with the person at mummy market.. she also said ask my lactation one to advise when i deliver..