About 2MB... You only need a different exe. (So you can look yourself how big that is. The Win98 fallback code wont add many bytes to it.
PS: Do you have IE5 installed?
Because the MSDN library says about SHGetFolderPath:
MSDN library wrote:
Minimum operating systems
Windows 95 with Internet Explorer 5.0, Windows 98 with Internet Explorer 5.0, Windows 98 Second Edition (SE), Windows NT 4.0 with Internet Explorer 5.0, Windows NT 4.0 with Service Pack 4 (SP4)MSDN library
So actually even Win95 should be supported, as long as IE5 is installed...