Upon launching "http:\\<EVservername>\enterprisevault\search.asp"  you get the below page with limited number of search parameters.

If you want to perform an advance level of search by specifying "size range" or "number of attachments" than

add "?advanced=1" to the search.asp link.

"http:\\<EVservername>\enterprisevault\search.asp?advanced=1"

If in case you want to perform a search on custom attributes than you need to append  "?advanced=3" to

the search.asp link.

"http:\\<EVservername>\enterprisevault\search.asp?advanced=3"