This is a common sentiment, but not really sensible. If you want to store (for example) another 64 bits worth of information, you would always be better off with 64 more bits of some strong hash than 64 bits of filesize.
http://www.mscs.dal.ca/~selinger/md5collision/
'Extension attacks' are something else, which let you turn one collision into more, or create valid hashes for combinations of unknown text plus a chosen extension – not find an initial collision. See:
http://en.wikipedia.org/wiki/Merkle%E2%80%93Damg%C3%A5rd_con...
The 'length extension' property can be helpful, once you find a collision based on 'random' nonsense, in extending that into two documents that are each meaningful-but-different and still colliding, as was done in this 2005 MD5 collision demonstration:
http://replay.waybackmachine.org/20050612011328/http://www.c...
Of course every extra bit that has to be matched makes collisions 'harder' but length bits are much weaker than other options, except insofar as they may already be available for other reasons.