Hi Chris
If you look on the Forum under:
Home > Technical > Trolley Dimensions for Roof Rack & Trailers
you'll see a diagram of key measurements taken from a Mk 2 Trolley which I used when buying my own trailer.
The Mk 1 was shorter but width at the X-bar of the A frame was very near the same. If you have a Mk 3 then best advised to check the length, I imagine the X-bar width is very near the same; but again best to check.
A potential problem with a shorter road base maybe ground clearance under the transom. I used a Mersea 275 to collect my boat when new and had use of a 250 for a couple of trips. Both were straight A frames on 8 in wheels and when coupled on the ball hitch inclined the boat down at the stern; to the point where crossing any ramps & speed bumps, necessitated stopping and checking clearance.
The road base I bought has a step down between the ball hitch and the apex of the A Frame, carries the boat level on 10 in wheels, and > 1 ft ground clearance at the transom.
If you have to use a low transom trailer a good tip is to put a piece of foam pipe insulation, the sort that's slotted to enable it to fit over domestic water pipes, over the transom lip before you put the under or over cover on the boat.
Hope this helps.