A .69 cal bore would take a .680 dia ball patched with a .010-.015 thick patch.
Dixie's 2008 catalog shows that they have these at $22.50/100 but I have several thoughts on this.
First off, Dixie seems to have gotten out of paying someone to cast some of the special sizes they used to have so they may not actually have them on hand.
Another thought about your pistol is it looks similar to many that were sold back in the '70'-80's that were intended more for hanging on a wall than actually shooting.
If it were mine, the first thing I would check is the hardness of the frizzen. To do this, try to file the frizzens face (the part the flint would hit). If a flat metal file makes a mark on it, it has not been hardened properly so it would produce few if any sparks when struck by a flint.
If the gun was made in Italy or Spain there should be proof marks on the barrel. If these exist on your gun it will be safe to shoot.
Some of these pistols were made in Japan and may not have proof marks. If this is the case with your gun it may be shootable but just how safe it would be might be a question.
If the frizzen is hard and there is a touch hole in the side of the barrel in line with the pan the gun should work but even so, I would keep my powder charges on the light side such as 15-25 grains of powder.
Dixie recommends a 30 grain FFFg charge for the .69 cal Pedersoli French AN XII pistol they sell and their "original load" data says that 35 grains was used in the original .69 cal pistols but unless your guns shows proof marks I would hesitate to use those loads.
Being a Flintlock, your gun must use real Black Powder. None of the new synthetic powders will work well in it.