I found something like this from GMAX is very comfy and one of the cheapest around. I have a big fat head also and most full faced helmets are too tight fitting, not GMAX ones. Best bet is to find a dealer who has GMAX and others and try them on before ordering. I would never buy online since my melon is oddball sized and want something not fancy, cheap, and comfy/roomy fitting - GMAX like this one fits my bill:
http://www.jazzmotorsports.com/gmax-gm17-open-face-helmet-gloss-black-p-312735.htmlI like the open faced design being able to open the entire shield in front in town driving to get more air and also to sip my drink in town at stop signs/slower speeds. A full faced helmet being modular flip up, also being much more expensive, will allow that also though.