I followed the suggestions in the Knowledge Base article Q105432 "HOWTO: Creating CD-ROM Folders in Developer 7 That Are Similar To CD-ROM Folders Built In Professional 6.x".
However the script does not realise that any of the files are going to be installed!
In the custom setup dialog (SdFeatureTree) the required size is 0 whatever features I select, and the "FeatureCompareSizeRequired" function says I have enough room to install on a full floppy.
Here's the relevant bit of my OnFirstUIBefore:
szTitle = "";
szMsg = "";
nResult = SdAskDestPath( szTitle, szMsg, szDir, 0 );
INSTALLDIR = szDir;
MsiSetProperty(ISMSI_HANDLE, "PROGRAM_DIR", INSTALLDIR^"Directory");
if (nResult = BACK) goto Dlg_SdWelcome;
When it then goes on to install I get an "Out of disk space" error saying that volume "a:" needs more space, but listing the requirement against volume F: