They have Extra Slim (slimmest), Slim (middle) and Classic (least slim). I'm not sure what you're talking about Ultra vs Extra Slim?
This weblink shows some dimensions that may help you: http://www.ctshirts.com/Formal-shirts-sizing-guide?cnt=sizing-guides
I wear an 18"collar and usually even slim shirts baloon out for me. Not so much the case with Charles Tyrwitt. Although I can only go up to 17.5" collar on the Extra Slim for now.
I misunderstood your statement. I thought there was an "ultra" slim line from CT
I did say that, sorry.