erm, can someone explain how a frame can be too long & too small to me please
If you are 6' but have overly long legs (36"), you may find that some stock frames may be right for leg length but too long for your torso length.
If they want an OCLV a 58 would initially be right size for a 6 footer, but because of the leg/torso ratio the saddle would be higher and the stem need to be shorter than normal. This would mean that extra spacers would be needed and the handling would be a little twitchier due to the stem length. On a bike with a carbon steerer the extra spacers would not be ideal.
So before buying your dream frame, invest in a proper fitting. Then see what allows you to achieve this position. I know that there are several dream bikes that I can't consider due to the geometry they have.