Andrew, you said:
"Firstly open up ID_Symbols.mcd, duplicated Wdw ID 1 to get Wdw ID 6. Symbol has TagSchema record attached with Window Record specified in TagType. Edited symbol to get h x w. Linked 'h' to FldlSpec: Window Height and 'w' to Fld2Spec: Window Width"
Your field specifications are OK, but you should link your text objects in the symbol to the Fld1Text and Fld2Text fields, not the Spec fields. The Fld#Spec fields determine what goes in the Fld#Text fields, which is what you should link to. Your Fld#Text fields should also have some default text value (even if it's only a space) in your symbol definition.
"...Now I am unclear how to proceed. I only get one field value to specify and do not want either a prefix or suffix."
If you are using a VA window object, it should already have a window record attached and the Window Height and Window Width fields should already be correctly filled out. If you are using a symbol, all you need to do is scroll down the list of fields in the field list until you come to these fields, then use the Selected Field Value field in the dialog and the Set button to set these values. That way the values in your Window data record (which will show up in your Window schedule) will always match what is on your ID tag.
Make sure the library file containing your ID tags is closed also.
I have modified the Window ID 2 symbol to take on height and width characteristics. I'd be glad to send you a copy of it. Email me at randerson@nemetschek.net to let me know.