What you're experiencing is the lack of blood returning through your neck to your body. The arteries are a high pressure system and get blood to your head despite the pressure on your neck. The veins however are a very low pressure system and it's hard to get the blood to return to your heart because the pressure in the neck.
In other words, your neck seal is too tight.
My drysuit materials said that if that happens I could stretch it by leaving a 2 liter bottle in the seal for a while. (I wuld start for 1-2 hours only, then work up if that doesn't solve it.)
Eventually, it should get better if you wear it enough. Hopefully you don't have a stroke by then.
My drysuit came with options to put whatever seal size you want on it for the arms legs and neck. You should be able to get a bigger neck seal put on the suit you have now. This allows for pencil-necks and muscle heads alike to have a nice, dry experience.
But to answer your question, I think the larger suits usually come with larger seals.
My drysuit didn't come with any parts I would be cutting.