A few years ago I changed my insurance to a cheaper one, and it was only when I read the small print it said that 4 wheel trailers were excluded. Under the 14 day rule I got a full refund, didn't know about the length though, so size does matter after all.
Mine is a boat trailer, (Seperate insurance for boat which includes the trailer and the engine. Infact, the boat could be in the water, the trailer on dry land and the engine in my garage... Sort that one out if there is a claim).
When connected to the car, the car insurance gives third party cover ie if you damage another vehicle or property.
Not sure which insurance you claim off for a breakaway while on the road, or if your caravan rolls down a hill in a field while camped....
You only find out how good insurance is (or not) when you have to make a claim.
If you think that's a problem.... have you read the small print for your breakdown or relay cover, you may be surprised.....