According to a comment on the page for the larger version, its reported dimensions are external and so I would presume the same holds true here. The descriptions for both products do mention that the thickness of the walls is 2mm (although the commenter on the dimensions mentioned 'around 2.2mm'), and so by doing the math, this means the internal dimensions here are (approximately) either 110x59x29mm or a tad bit less. In my opinion, such discrepancies shouldn't matter too much if you can get the enclosure before finalising any design. When putting a rough sketch together, very exact precision down to the millimetre would only be truly needed if one were to be really cramming a lot of components inside, which isn't generally advisable in any situation, especially the first iteration.

A final word of advice: the dimensions shown by the pictures don't quite match the descriptions in any case, one reason to contact customer support to clarify if such information is absolutely crucial, though in prototyping it typically shouldn't be.