I have used a 12”x9” granite block to fettle the #4 I use bit recently acquired a #5 that belonged to my great uncle. It looks good, and I’m looking forward to cleaning it up, but I assume that I need a flat surface that is longer than the plane sole. Is that correct? Thanks, John
Yes, assuming you're talking about using the granite block (surface plate I presume) as a backing for abrasive paper of some description. You'll want to use the diagonal (15 inches) which is just about as long as the sole and limit your overhang during each stroke to a couple inches. There may be some tendency to produce a concave surface, so you should check the sole for progress frequently. You can reduce high spots more quickly (perhaps on the ends if concavity appears) by concentrating your downward pressure above those points, probably one at a time.
Do establish a feedback loop which allows you to monitor your progress and get a feel for the effectiveness of your technique. Feedback here meaning measuring the flatness against a reference every couple minutes or so. Unfortunately your best tool for that may be covered with sandpaper just when you need it (the surface plate itself), but perhaps you have another straight-edge available that will suit. Use rocking (or the lack of it in the end) and perhaps feeler gauges to locate and measure spots not making contact.