The banner letters being towed in that photo are made by Gasser in USA. They come in two sizes, either 5 ft or 7 feet high. I don't know the vicinity but would guess that these were 7 ft letters which can easily be read when flown at and above 1000ft. If the glide clear was possible I am sure the flight was legally flown - assuming of course the aircraft was approved for banner towing, and the number of letters being towed were in accordance with the operators towing manual.
It is a non event from an aviation perspective - only of interst to footy fans.