hi Dave,
the difference in case of the rates between a cabby and a reliable english speaking pre-set up with IBK is 0-5 bucks per ride(always per ride rates, not per person), IBK Tourist Services is always using minivans, so space for big guys is no problem, you will fit in.
you can contact Bronly from IBK for a actual quote on
ibkservicios@gmail.com
i use his services very often myself for my business, never had any prob or complain.
have a great time on the Isle
Mike