According to this page, Dorj is as large as a seal.
In this picture, he appears to be about twice the size of the children. This page also describes him as being like a seal, as does this page.
Various images of him show him in varying sizes, so there doesn't seem to be one set size, though he is always described as large. (Ex. 1, Ex. 2, Ex. 3)
These are action figures of of the characters giving one version of the relative size:
In the picture, Dorj is about half the size of Rin's body, but her head is disproportionately large and this may not be canon in terms of proportions, so it is difficult to tell.
As a whole, half the size of Rin seems to be a reasonable estimate, but there doesn't seem to be an official size beyond "large" and, sometimes, "large as a seal".