Erk! Holy cow. I don't think they have "too big" exactly, but I would expect eventually they will no longer be able to scavenge enough food, and will either starve or eat something you didn't want them too out of desperation.
Makes me think of my discovery today of 3 asterina-type stars(pale blue-gray with reddish dots on the top center) that are almost penny sized. Never heard of them growing larger than pencil erasers, but my little nano is pretty mucky...