Stretching the seal to make it fit will stress it and cause it to perish quicker.
Latex neck seals are designed to be trimmed to fit, that is what the rings on them are for
Just trim off one ring at a time, between the raised ridges, with a sharp pair of scissors. Make sure there are no "notches" on the edge to act as as starters for a tear.
Only do it one ring at a time, no matter how tight it seems to start with. Try it on between each trim. It will slacken off slightly with time anyway, so for a new seal, so long as it is not making you gag and your eyes bulge it is probably ok. You can always take another ring off later
cheers
dave
drysuitrepair.co.uk