Grape agate is a marketing name for tiny purple quartz crystals that have a round shape, purple color and a cluster that resembles grapes. Geologically speaking, these are not really agates, but instead more likely amethyst, or even more accurate - "botryodial" (round) amethyst.
The "grapes" of these specimens are tiny. They range between about two and eight millimeters across. Although purple is the most common color leading to the "grape" name, some specimens have grapes that are white, gray, green, or blue.
Please look closely at the photos and video - they show each of the specimens in various angles so you can get a good view of what the pieces look like to help make your selection.