The best I can do is to give you a body length range for the winged lifestages of those Ephemera species inhabiting North America. Mayfly body length is measured from the front of the head to the end of the abdomen, and does not include length of the tails. For Ephemera, body length ranges from 10-30 mm. In North America, one of the Ephemera species is quite a bit larger that the others. That is the reason for the large range of body length. Sorry I can’t be of more help, but I don’t (yet) have detailed descriptions for European mayflies.
Thanks!!! It helped a lot. Now, I go for superrealistic tying of an Ephemera.
Taxon, you is the best! Thanks again!