A quick reconstruction outside gives us a simple right triangle with the hypotenuse being the rear of the trl.
And from full size measures (a ■■■ packet ain`t big enough) one side at 20cm.
Deploying our old friend Pythagorus
The third side (the apparent width of the trailer) is sq.rt of, 2.55sq minus 0.2sq.
Equals 2.542m
Due to the offset of the observer the trailer appears less than 1cm less than it actually is.
Open to correction on the above.
Just to counter the first argument I can see, I moved the pov to 10m away rather than the 15m I estimated.
And the offset to 2m away from side of trl.
Both are IMHO the extremes of what we see, and will give the greatest error. This gives a side to the triangle of approx 40cm.
False reading of 2.52m an error of about 3cm
The technical term, in estimates like this, ( not in rocket science) is
Sweet Fanny Adams.
The trailer, scaling its width to its ht, is about-ish 2x 2.55m
Or
About-ish 2x 8ft 4inches
More than is legal, but we are only dealing with estimates.
Using again the lexicon of technical terms, where we found Sweet F A, and prefixing that with a more common usage,
A blind man, can see that`s too ■■■■■■■ tall.