Size is as-packaged, or the diameter of loose marbles. ("D" is the diameter of round items other than marbles.)
This tube of glow-in-the-dark 5/8" marbles makes a wonderful night-light! The Marbles need to be "charged" before they will glow - shine light on them (or hold them close to a light bulb) then when you turn off the lights they will glow like crazy! Kids will love to charge them before packing them back up and taking them to bed.