Dimensions - 16cm from one hexagonal face to the opposite. And 26cm long bellows folded, including handles
The imperfect action is buttons getting stuck down, they can be released by hand, and is pretty infrequent, I'm sure a service from a competent concertina techy would sort.
Airtight, I think its most likely the bellows, not quite sure how you diagnose if its this or pads, but the thing has enough air to play
Comparison to Rochelle - well I've never played a Rochelle and so the forums could answer this for you I suspect, but looking at the Rochelle on line its clearly made in a very similar way, I should think its a shop specific model of a Stagi, but its not quite the same model, the one I have is a bit fancier with the in lay and ends, but mechanically I would guess they are pretty close.
USA postage looks about £60
Hope thats some way to answering you questions