What's the biggest organic molecule that could have a smell?
I'll quote from$\ce{^{[1]}}$:


  The general requirements for an odorant are that it should be
  volatile, hydrophobic and have a molecular weight less than
  approximately 300 daltons. Ohloff (1994) has stated that the largest
  known odorant is a labdane with a molecular weight of 296. The first
  two requirements make physical sense, for the molecule has to reach
  the nose and may need to cross membranes. The size requirement appears
  to be a biological constraint. To be sure, vapor pressure (volatility)
  falls rapidly with molecular size, but that cannot be the reason why
  larger molecules have no smell, since some of the strongest odorants
  (e.g. some steroids) are large molecules. In addition, the cut-off is
  very sharp indeed: for example, substitution of the slightly larger
  silicon atom for a carbon in a benzenoid musk causes it to become
  odorless (Wrobel and Wannagat, 1982d).
  
  A further indication that the size limit has something to do with the
  chemoreception mechanism comes from the fact that specific anosmias
  become more frequent as molecular size increases. At the “ragged edge”
  of the size limit, subjects become anosmic to large numbers of
  molecules. An informal poll among perfumers, for example has elicited
  the fact that most of them are completely anosmic to one or more musks
  (e.g. Galaxolide® mw 244.38) or, less commonly, ambergris odorants
  such as Ambrox®, or the larger esters of salicylic acid.
  
  One can probably infer from this that the receptors cannot accommodate
  molecules larger than a certain size, and that this size is
  genetically determined (Whissel-Buechy and Amoore, 1973) and varies
  from individual to individual.


References

$\ce{[1]}$: "Structure-odor relationships: a modern perspective", by Luca Turin (Dept of Physiology, University College London, UK) and Fumiko Yoshii (Graduate School of Science and Technology, Niigata University, Japan), which appears as chapter 13 of: Handbook of Olfaction and Gustation. Richard L. Doty (ed.). 2nd ed., Marcel Dekker, 2003.

$\ce{[2]}$: Ohloff, G. (1994) Scent and fragrances: the fascination of odors and their chemical perspectives [Berlin, Springer]. 

$\ce{[3]}$: Whissell-Buechy D. and Amoore J.E.(1973) Odour-blindness to musk: simple recessive inheritance Nature Sep 21;245 (5421):157-8.