I think it's a bit facile to say the plane was the wrong size. Planes can be any size. The Embraer sells. So does the A380.
The question is why did the BAC 111 and Trident not succeed whilst the 727 and 737 did.
I suspect this was more to do with politics and national jealousies than merits of the planes themselves.
Just to correct this impression that the Trident was a dog - I remember it going up like a rocket on occasions.