Guinea pig costume sizing: The guinea pig models wearing the costumes in my photos are full grown (approx. measurements: 8-9" from back of ears to their bottom, 10-11" waist) that will help you judge the size of the costumes.  Most costumes will fit an average size adult guinea pig, but measure your guinea pig first to be sure it will be a proper fit.  If the guinea pig is too young/small, they will walk out of the costumes.
Lop Rabbit costume sizing: The rabbit shown is an adult Holland Lop. She is approx. 3 pounds.