Starburst Memorials (claiming to provide a service to send one’s creains into space) estimates that the cremains will weigh 3 kilograms (just over 6 1/2 pounds).
Assuming that the ashes weigh less than water (at just over 8 pounds/gallon) and would, therefore, take up somewhat more volume, the differences in volume for cremains might range from about a half gallon (at 3% of a 100 lb. person) to two gallons (at the older rate of 5% for a 300 lb. person.)
I did not find anything that provided actual volumes of cremains, although this on-line urn vendor notes on several of its products a volume of 200 inches (just under .9 of a gallon).