This helicopter measures a little over 18 inches from the front of the cockpit to the tip of the tail, and about 17 inches from tip to tip of the rotor blades.
Unfortunately, I don't have the manual for the set, #1721, so had to build it by guess and by golly from the picture on the box and a similar picture in a catalog from the 1994 era of the set. I'd sure like to get a copy of the instruction booklet for this set. Having the box and catalog exclaim "20 models" just pours salt in the wound.
Erector Junior is Meccano Junior outside the United States, and at the time this set was made, all the Erector and Meccano products were made in Calais, France. Current sets are made in China, for Meccano, which is still in Calais, and are imported into the United States by Brio.
The body of this helicopter is made up of hollow plastic double or quad "blocks," as were constructions from most Junior sets of the era. These "blocks" seem to have been dropped from the most recent sets, taking them even further from the concept of block play.