Facts and interpretation.

No one has claimed OP s carry on was too big.
LH claimed it was too heavy, which is not proven, and is disputed.
In any case OP expressly explained his carry ono contained laptop (s), camera(s) etc, but was not given the option to repack/redistribute/rearrange.

Im not even taking into consideration the attitude and (missing) customer service.

Contrary to what seems like common belief IDB is not only by overbooking.
Article 2:
"(j) "denied boarding" means a refusal to carry passengers on a flight, although they have presented themselves for boarding under the conditions laid down in Article 3(2), except where there are reasonable grounds to deny them boarding, such as reasons of health, safety or security, or inadequate travel documentation"

http://eur-lex.europa.eu/LexUriServ/...61:EN:HTML:NOT

Interpret reasonable.

The different national organs can be found here;
http://ec.europa.eu/transport/passen...ent_bodies.pdf

OP might want to contact
Luftfahrt-Bundesamt (LBA)
Hermann-Blenk-Str. 26
DE - 38108 BRAUNSCHWEIG
Tel. : +49 531-2355-100
Fax : +49 531-2355-707
[email protected]

On my own behalf, and clients I have won far more difficult battles than this. I feel OP has a pretty strong case based on case law/practice, with a well written and well documented claim.