[surbat6]

The standard size for U.S. percussion muskets is (ready for this?) musket caps. They're often called "top hat" caps, because they have four flanges around the open end that make them look like little top hats. They are larger than the #10 and #11 caps most of us are used to. I have a reproduction pecussion carbine that came (2nd hand) with two nipples - one for musket caps and one for #11's.
If you remember the top hat shape of the caps you used before. It's simple which ones you need. None of the smaller sizes have the "hat brim".
If your grandfather's rifle is an original 1861 Springfield musket, PLEASE clean it carefully before putting it away. It's fine (as far as I'm concerned) to use an old musket that's been checked out as safe to shoot, but keep in mind that these guns are always increasing in value...unless somebody doesn't care for them and they rust all to blazes.
If, anywhere on that musket, it is marked something like "use black powder only" it's a reproduction (black powder was the only kind there was when the originals were new), and the readon you should clean it thoroughly is out of respect for your grandfather's property.