Actually. . . you are right.
The maximum size of ship that can be escorted through has to be less than the jump ship and the jump drive rating. For example, if you have a 28 000 tons jump ship, you will only be able to open a wormhole for ships of 28 000 tons or less, regardless of your jump drive efficiency.
Luckily I am just playing my first game to learn the game and therefore don't mind correct-SMing mistakes like that.
Better to make that mistake now!
On a different note. . . if I want my (now correctly sized) ships to function as permanent gate opener, do I have to station 1 ship on both sides or is 1 ship on any side enough?
I guess it is 1 ship on both sides, but I could of course use just one ship and micromanage it from side to side. As if there wasn't micromanagement already