Well, after a little searching on Lunch in a Box, I figured out about how big my bento box should be. She has a handy calculator on there, and using that this is what I figured out:
I am in my 20's, so I should start with a 600 ml box for calculation purposes. I am shorter than average, so I should subtract 100 ml, and I have a low activity level, so I subtract another 100 ml. (I think... either way I am trying to lose weight, so a smaller box isn't a bad thing.) So my bento box should be about 400 ml.
So I just went and checked the box I used yesterday to pack my lunch and Ellie's lunch together and... it's like 1600 ml. HUGE. And not huge at the same time. But that explains why we only ate about half the food in it and felt plenty full. Weird. So that must be the family size box. Good to know.
A 400 ml. box is small. Very small, by my normal lunch packing standards. But if I could fit a whole lunch in that teeny tiny box, it would fit in my purse. That would be convenient! So I'll try again sometime soon. Maybe packing my lunch alone this time to see if I can do it. Right now I'm off to sort through the Tupperware and see what containers I already have that will fit the bill. How fun!