A variable size nanoscale particle sieve with openings between 2 and 50 nm has been used for determining, selecting, and manipulating the size of large liquid helium clusters in the range from 7 x 10(3) to 3 x 10(6) atoms. The variable openings of the sieve are obtained by rotating a nanofabricated free-standing transmission grating with a 100 nm period and 50 nm wide slits around an axis parallel to the slits. The new nano;sieve can be applied to clusters of various species as well as to large molecules with sizes down to 2 nm.