Here's what I found from the State of Connecticut
here and in blue below.
We were just on it back in August in our minivan. I seem to recall some low overpasses, though I don't remember reading the posted heights. I don't remember seeing anything but cars and vans. In the text below, it plainly states vehicles over 8 ft. high are prohibited. The length and width restrictions would probably also keep most RV's out. From what I remember of the service stations, a large rig would NOT be able to refuel at them.
What is allowed to travel on the Merritt Parkway?
The following are prohibited from entry upon and use of the highway right-of-way of those limited access state highways designated as parkways:
(a) commercial motor vehicles;
(b) trailers;
(c) all towed vehicles except as provided in Section 14-298-240;
(d) buses;
(e) hearses when part of a procession or cortege;
(f) vehicles bearing other than passenger, camper, taxicab, vanpool or hearse registrations and those vehicles bearing combination registrations which have a gross weight in excess of seventy-five hundred pounds;
(g) vehicles whose dimensions, including any load, exceed one of the following:
length – twenty-four feet
width - seven feet, six inches
height – eight feet
Permits for entry upon and use of the parkways by vehicles normally excluded therefrom may be issued by the commission when, in its opinion, the interests of public necessity are served thereby. Such permits shall be in writing and shall specify the period of validity of such permit, the parkway or portion thereof and the identity of the vehicle concerned. A copy of the permit shall be carried in the vehicle. Apply for a permit
Most RV's exceed the dimensions they give. Nice road, though, so in a car it's a great way to get down to NYC from points north east.
~Rick