High Quality 0.6mm thick Drysuit/Top replacement latex neck gasket.
Glue in diameter is 26" which is pretty much the standard for most all Drysuit/Tops.
Neck size is L (large) which is NRS describes as ideal for 15-18" circumference necks. My neck is slightly smaller in the 14" neighborhood and I find this size ideal. If your neck is smaller than than that you will appreciate the extra comfort and still find it to be adequately dry.
Glue in with Aquaseal which you can find here on the site, or be a cheap dirtbag like us and use Shoe Goo. If you need help fire us an email and we can send you some vid's as well as some tips from our personal experience.
Also you should consider picking up a bottle of 303. The lack of using 303 may be why you are here in the first place.