Is your bike an FXSTC or straight Softail Standard?
Specs show a 19" wheel as the front with a 100/90H-19 for the Standard and Custom.
If you're asking if a 21" wheel will fit on there with the stock fender I would have to say I don't think it will. You're increasing the diameter and making the rim 1" closer to the fender. Any tire whould have to be a low profile tire to make up the difference. In the auto world you can get away with it.
Hopefully I'm understanding your question correctly and my 2007 model year specs are correct, but I could be misinformed. I thought all FXSTC's had 21" fronts with the exception of the Heritage, Fatboy's etc.