In liquid measurement, the bowls hold 6 fluid ounces when filled to the brim. Solids and semi-solids such as yogurt have different forms of measurement, so it really depends on what you want to put in them.
These are pretty heavy duty glass dishes. In a lunch box, I think they'd be safe without breaking.
Also, unless the glasses are held upright at all times, I would not reccommend putting anything too liquidy such as dressing or sauces. It might seep through.
Do you perhaps mean "leaK" proof? It would be an easy typo to make.
If you really meant "leaD", I'd say it's highly unlikely to contain lead. If you meant "leaK", I'd be a little reluctant to try it; the lids fit snugly, but it's easy to not get an edge down firmly. I use them for yogurt bought a liter at a time and rarely transport a full one anywhere. I like them enough that when I broke one recently, I ordered a new set so'd have exactly as many as I need. Yes, they are glass and they can break; I dropped one on a laminate countertop from maybe 8" and it broke into a number of pieces of widely varying sizes, some quite sharp. Maybe not for a kid's lunchbox.