There are many different species of isopods, and they attain drastically different sizes.

For example, the deep-sea giant isopod (Bathynomus giganteus) can grow up to 20" long. On the other end of the spectrum, aquatic isopods from the taxonomic family Microcerberidae are as small as 300 micrometers! That's about 0.01", or 1/100".

As far as the isopods we offer, here are their maximum sizes in order from largest to smallest:

Giant Canyon Isopods (Porcellio dilatatus): 1"
Granulated Isopods (Armadillidium granulatum): 1"
Lottery Ticket Isopods (Porcellio scaber): 3/4"
Dalmatian Isopods (Porcellio scaber): 3/4"
Calico Isopods (Porcellio scaber): 3/4"
Dairy Cow Isopods (Porcellio laevis): 3/4"
Japanese Magic Potion Isopods (Armadillidium vulgare): 5/8"
Powder Blue Isopods (Porcellionides pruinosus): 1/2"
Powder Orange Isopods (Porcellionides pruinosus): 1/2"
Candy Crush Isopods (Porcellionides pruinosus): 1/2"
Panda King Isopods (Cubaris sp.): 3/8" (almost 1/2")
Dwarf White Isopods (Trichorhina tomentosa): 3/16" (almost 1/4")
Costa Rican Dwarf Purple Isopods (Trichoniscidae sp.): 1/8"