I agree with ILTMIMI - I'm Victoria, but am only known as that if: a) in trouble with my mother, b) At the doctors, or c) at a job interview....
My mum chose names for me and my sister (Rebecca) that had a vatiety of alternatives in case we didn't like our names when we got older - I could chose to be Victoria, Vicki, Vix,Vic, Tori etc etc, although seem to have settled on Trix (only by close friends and family) and am Vicki everywhere else.
Mum was also concious that being girls, if we married a man with a short last name that our first names would balance things out - if I was Jane and I married someone with the surname Bell, I would be Jane Bell - very monosyllabic, whereas I would be Victoria Bell, which she thought would be a bit more interesting. (Obviously apologies to all the Jane Bell's out there!!)
When it came to naming my DS, DH and I chose a name that we both adored, but that could had alternatives...when he grows up, he can be Joseph, Joe, Joey etc. Funnily enough, the name that's stuck for me is Joster?!?! Very odd I must admit
At the end of the day, naming your DC is a very personal thing, but you've got to accept that kids will always find a way of shortening it - my DH is called Kieron, which MIL thought no-one would be able to shorten, however at school, he was known as Ron, Kez, Ques-ce-sez all sorts!!
I think give your DC a nice, solid name for them to use at formal things and leave the rest up to them!