Only very slightly larger - in the field, it's academic. The biggest difference is that rat burrows have spoil heaps outside, but also they tend to be higher up the bank and have interconnecting runs between them.
Water Vole burrows have neat entrances, often just near water level (and below it, and above it in case of flooding). There are sometimes, grazed areas of grass near w-v burrows, or flattened areas where a fat vole bottom has sat.