you want to find what ? free space ? (sorry, it's a joke
)
the "emuNAND" is an option to install and launch WAD to SD or USB.
you first need to make a copy of your Wii memory to SD or USB, then use that copy instead of your real console's memory.
I don't know about wiiflow, I'll tell you with USBLoaderGX :
USBLoaderGX > features > dump NAND > Full
USBLoaderGX > top menu > 4th icon > enable "channels on emuNAND" to list the channels installed on the copy (SD or USB)
try to launch a game from here to see if it works (not 100% compatibility!)
To add new game to the copy, instead of the console memory or official SD menu :
USBLoaderGX > features > emuNAND Wad manager > file/folder > install/uninstall
if you find a game not working (you will have games not working ! users often try to launch a non working games FIRST ... don't try castlevania !! it's not working), then you will need more setup.
we will see later how to setup "neek2o" to fix the non working games.