Size is as-packaged, or the diameter of loose marbles. ("D" is the diameter of round items other than marbles.)
This assortment is a great way to add some variety to your stash. I opened a net to check and found about half the marbles were unnamed oddball varieties. There was 64 marbles total, and eight of them shooters. One of those shooters was an oversized boulder! (The count and mix of sizes will vary). It includes a colorful header card printed with game rules.