What are the dimensions of the biggest compartment? Would it fit a small thermos?

Hi there, here is the dimensions here: Lunchbox dimensions 260mm (W) x 220mm (L) x 60mm (H). The compartments come in varied sizes so it is likely the thermos would fit. If you are unsure, feel free to contact your local store to see if you can try one out for yourself. Here is a list of our Store Locations here: https://www.babyonthemove.co.nz/store-locations/. Hope that helps