Just type in 500 MB, but that doesn't work very well. If you add other words to the search, like the title, you'll get search results for the title that don't match the file size.

Probably the best way to search by file size, is to do the search, then double click on the "Size" column, and that'll put the largest files at the top of your search results.

__________________
If it ain't broke, fix it till it is!