Let your gun tell you what you need to know. With a coupon from Harbor Freight, you can get a quite acceptable digital caliper for $10. Everyone should have a caliper for measurements such as this.
With the large bore 10 gauge with a bore diameter of 0.775", a ball that is one heavy patch thickness less would be a good start. I would suggest a 0.750" ball and a patch of 0.022" or a paper wrapped cartridge with three layers of paper around the ball. Use of tow for wadding would be good in lieu of a cotton patch. Get some ball for trial before spending money for a mold that may not be suitable.
I have a Long Land Pattern King's Musket (Brown Bess) with a bore diameter of 0.770". Not really a 10 gauge but close. I regularly use a ball of 0.715" with a wrapped paper cartridge. I need to try the 0.735" mold that I have been waiting for a good time to cast some ball. The 0.715" ball works for me for most of my live fire uses.
You have a wide range of choices for selecting a ball diameter for loading your gun.