Hello Ken,
Most of the ISOGEN symbols can be scaled using Symbol Editor. Check if symbols are User Definable in C:\Program Files (x86)\Bentley\Plant V8i\Help\Isogen\Symbol Keys.pdf (path in 64 bit windows). More detailed information on Symbol Editor toll can be found here C:\Program Files (x86)\Bentley\Plant V8i\Help\Isogen\Symbols Editor.pdf.
I hope this information will help.
Regards,
Artiom