Here is a paper utilizing surface acoustic waves for the sizing of binding proteins within your weight range: http://www.ncbi.nlm.nih.gov/pubmed/24943453 . Another paper uses a microfluidic nozzle to spray a specific volume of fluid onto a quartz crystal microbalance, where the dry mass of the solutes is weighted after solvent evaporation: http://scitation.aip.org/content/aip/journal/apl/105/21/10.1063/1.4902131

In this paper a microfluidic diffusional sizing approach is combined with an amine reactive latent label to size proteins in solution:

https://www.nature.com/articles/nchem.2344

However, the approach determines hydrodynamic radius rather than MW, which doesn't always correlate depending on how the protein is folded. In theory though, it should be possible to size anything from single amino acids and small peptides (