You mean total size of all the files in that folder?
If you have the ifsFileSize API I wrote or using the STAT api in the C runtime, you can determine each file's size within a directory. I've also written articles on recursing directories and reading each file name as if you were processing a database file's records. Then you could apply the STAT APi to each file that's read. But there's also Directory/Folder overhead.
What is your specific goal?
Gosh, what a dope I am!
Did a BING/GOOGLE Search for IFS Folder Size and found the Command QRYIFSLIB from