The only way to sex a croc of this size is to pop it. Pop sexing is in no way a sure method of sexing unless you get very lucky and the penise pops out on a male.
I myself dont pop sex croc's and most other croc keepers I know also wont pop sex a animal as it is very stressfull on the animal, care must be taken when doing this because you could easily hurt the animal and on top of all of this as I said this sexing method is in no way accurate.
I dont advise pop sexing but if you still wish to do so detailed instructions can be found in the croc faq at www.crocodilian.com
The odds of a common caiman growing to be 2M are very slim to none, it is possible to get a male of this type to a size of 6-7ft but that would be a rare situation.
I think with a male of this species 5ft is a nice large size animal and 6ft would be a VERY large and rare size specimen.
In my opinion the sex of a crocodilian is of little importance unless you are planing to breed the species, it is also my oppinion that crocodilans should not be sexed until they are about 3ft in length unless it absolutely needs to be done under very extreme circumstances.