For objects with Content-Length headers, it can be done by defining a Content Policy Language (CPL) entry. The following are a few examples :
; Do not cache objects smaller than 4000 bytes
<Cache>
response.header.Content-Length="^[1-3][0-9]{1,3}$|^[0-9]{1,3}$" cache(no)
; Do not cache objects smaller than 6000 bytes
<Cache>
response.header.Content-Length="^[1-5][0-9]{1,3}$|^[0-9]{1,3}$" cache(no)
; Do not cache objects smaller than 30000 bytes
<Cache>
response.header.Content-Length="^[1-2][0-9]{1,4}$|^[0-9]{1,4}$" cache(no)