Set file size limit for video on Canon 5D mark iii
The 4 gig limitation is there cause most memory cards are formatted with FAT32 which can't handle files larger than 4 gig.

Refer to Manual page 234, http://gdlp01.c-wss.com/gds/8/0300007348/05/eos5d-mk3-im7-en.pdf

The only way to limit the file size, is to limit the recording time.
You can always do the math and find out how many minutes to record to hit 2 gig.

EDIT: the link no longer works.  Here is a screenshot of the page -