In order to know what size urn you will need for your pets ashes you will need to know the approximate weight of your pet. After knowing this you can decide what size urn your pet will need.
Allow 1 kg of body weight to 35 ml of urn volume, e.g. a pet weighing 10kg will need an urn capable of holding 350ml or a pet weighing 20kg will need an urn capable of holding 700ml.
This formula will be correct in almost all cases, but because of different cremation processes and different bone density of your pet the final ash volume may vary slightly.