What size did you get? Hooded vests are typically sized for the vest, but the hood should be comparable.
I have an 3 mil XL elastiprene merino hooded vest and the hood is a little tight but is OK. Pinnacle does not seem to have a head size chart for their hoods online, but if you go to a Pinnacle hood on the Leisure Pro site and pick the size chart, it will give general guidelines for hood sizes vs. head measurements.
You can also call Pinnacle, their customer service is great. I do believe that you can trim the hood, but I would talk to someone first as I believe that you would only want to trim if the hood covers too much of your face.