If you're in the UK or Europe then there isn't one standard trolley; there have been three.
Mk1 - short A-Frame, short gunnel arms, and short handle. Very easy to ground the transom, perfectly balanced for moving the boat around the boat park, good for roof topping, and easier to fit into a Mersea multi-stack trailer.
Mk 2 - longer in: A-Frame, Gunnel Arms, and handle. Much less likely to ground the transom, needs care roof topping on shorter roofed cars, less likely to fit the shorter road base. There is a dimensioned diagram of this trolley in other threads in the same vein on the Forum. I do recall the length from front holding pin centre to front of axle is 250 mm. This is the dimension referred to on road bases when the say it's a 225 - 250 or 250 - 275, to be sure I'd advise you to check.
Mk 3 - I'm not so sure on this version but I believe it is the same dimensions as the Mk 2 but with Gunnel arms that can be adjusted in height between the Mk 1 (low) and Mk 2 (high).
The holding pins on the Mersea trailers, as they are on other makes, are adjustable over a limited range; what is not usually specified is the maximum width of trolley A-Frame.
I carried a Mk 1 home on a Mersea 250 - 275 OK. I've also carried the Mk 2 on one.
Unless you know exactly which Mk # trolley you're going to be trailing then I suspect a Mersea 225 - 250 maybe too short.