I present some work in progress which axiomatises $(2,1)$-categories that have a notion of size attatched to them in the form of a class of discrete (op)fibrations in the $(2,1)$-category satisfying some conditions. This motivating example of a class $(2,1)$-category is $\mathbf{GPD}$, the category of large groupoids together with $\mathbf{Set}$-sized discrete opfibrations. The small objects here are the small groupoids. Other examples of note include Grothendieck $(2,1)$-toposes (otherwise known as a $(2,1)$-category of stacks) as well as a realizability example of groupoids internal to the category of assemblies over a partial combinatory algebra. The punchline of such an axiomatisation is that the internal language of the sub-$(2,1)$-category of small objects of a class $(2,1)$-category is a $1$-dimensional model of Martin-Löf type theory. This can be seen as a higher dimensional version of an important theorem by Awodey-Butz-Simpson-Streicher in the $1$-dimensional setting.