they are Win32 API functions. $findfile is an mIRC script wrapper for those functions (mIRC just calls them for you). aside from a dll you cant access them from a script (perhaps using a com object however).
If you plan on making a dll (think there is one already on
www.mircscripts.org) look into FindFirstFile,FindNextFile and FindClose functions in the MSDN
I dont see a method for searching based on filesize but i think ext based searches are possible.
