Thank you for posting your questions, Jill. Please PM me.
They are reallly nice, aren't they!
I only ever saw them for a brief time from only one vendor. I think they cost too much when I bought them so they did not restock. My loss, your gain.
I would call them extremely airtight. When I remove the outer lid, it does not simply fall off, I can feel the vacuum as I pull. But still it is smooth. The inner lid is added insurance in Japanese fashion. They are as airtight as any canister I have ever used.
How much do they hold, that would depend on the tea. The small one will hold 100 grams or 4 ounces of most teas.
The larger one will hold significantly more if your pour the tea right into it, but I usually put the vendor bag right into the canister. So this way, it also holds around 4 ounces. Or up to 8 ounces if you pour the leaf into it.
I never washed them per se, they never needed it, even the ones I am not selling which I have used for years ... just an occasional wipe. So, I cannot answer the dishwasher safe question, sorry.