Terrestrial isopods in the hobby range from species you can barely see without a magnifying glass to chunky animals that fill the palm of your hand. The size difference between a dwarf white (Trichorhina tomentosa) at 3-4mm and a full-grown Porcellio hoffmannseggii at around 20mm might not sound dramatic in numbers, but in practice they're almost different hobbies. What works for one barely applies to the other.
The dwarf end of the scale
Trichorhina tomentosa (dwarf whites) are probably the most commonly kept small species. They're tiny, pale, and they reproduce rapidly through parthenogenesis, meaning females can produce offspring without mating. You buy a handful and within a few months you have hundreds. They're the workhorse cleanup crew species for bioactive terrariums because they're small enough to avoid most predation from reptiles and amphibians, and their population self-regulates to available food.
Other small species include Nagurus cristatus (dwarf striped) and various Trichorhina relatives. These all stay under about 6mm as adults. Their care is straightforward: they want consistent warmth (20-25C), high humidity, and they'll eat leaf litter, decaying wood, and whatever organic waste is available.
The appeal of dwarf species is practical. They fit in small enclosures, breed fast, and work brilliantly as a functional cleanup crew. They're not display animals. You won't be showing off your dwarf whites to visitors because your visitors won't be able to see them without getting their face right up to the glass.
Mid-range species
Most of the popular hobby species fall in the 10-18mm range. Armadillidium vulgare (the common pill bug, around 15-18mm), Porcellio scaber (rough woodlouse, 12-16mm), and Armadillidium maculatum (zebra isopod, about 15mm) are all solidly mid-sized. These are the species that most people picture when they think of keeping isopods. Big enough to see clearly and enjoy watching, small enough to keep easily in standard tubs.
This size range is where you find the most morph variety. Porcellio scaber alone comes in Dalmatian, orange, calico, and several other colour forms. Armadillidium vulgare has magic potion, T-positive albino, and various other morphs that have driven prices up considerably from what people used to pay for what was, not that long ago, just a garden woodlouse.
Care for mid-range species is what most general isopod guides describe: substrate with a moisture gradient, leaf litter, calcium (always calcium), moderate temperatures, and decent ventilation. The standard advice works because it was written with these species in mind.
The large end
The biggest terrestrial isopods commonly kept in the hobby are the giant canyon isopods (Porcellio dilatatus) and the impressive Porcellio hoffmannseggii, both reaching about 20mm. Porcellio laevis (dairy cow isopod) is also on the larger side at 15-20mm and has the bonus of being one of the fastest-breeding large species.
There are also the giant Spanish species like Porcellio magnificus that can exceed 20mm and have been increasingly available in the European hobby. These are proper display animals. You can see them clearly, watch them interact, and they have enough presence to justify a dedicated enclosure that you actually look at.
Larger species tend to need a bit more food, more calcium (larger exoskeleton, larger moult), and sometimes a bit more space per animal than smaller species. Porcellio laevis in particular is protein-hungry and will cannibalise weak or moulting colony members if protein is lacking. Keep them fed and this isn't usually a problem, but it's worth knowing about before you set up a colony and wonder why numbers aren't climbing.
The deep-sea giants (a digression)
People always ask about Bathynomus giganteus, the giant deep-sea isopod that can reach 50cm and looks like something from a science fiction film. These are marine animals that live at depths of 170 metres or more. You cannot keep them in a tub in your bedroom. They're related to terrestrial isopods in the same way that lobsters are related to woodlice: same order (Isopoda), completely different suborder and lifestyle. The deep-sea giants are fascinating, but they're not relevant to the hobby beyond being a good conversation starter.
Choosing based on purpose
If you want isopods as a cleanup crew in a bioactive terrarium, go small. Trichorhina tomentosa are the standard for a reason. They establish fast, breed readily, and most reptiles and amphibians can't eat them quickly enough to outpace the colony. If your reptile is particularly small (a juvenile gecko, for example), even Trichorhina can be supplemental food, which is fine.
If you want isopods as display animals in their own right, go bigger. Porcellio laevis (especially the dairy cow morph, white with dark spots) or Armadillidium maculatum (zebra) are attractive and visible enough to actually watch. The Cubaris species (rubber ducky and similar) are mid-sized but their unusual shapes and colours make them popular display species despite being slower to breed and more expensive.
If you want to breed for sale or trade, consider what your market looks like. Dwarf species sell in bulk for bioactive use. Mid-range morphs sell individually or in small groups to collectors. Large or unusual species command higher per-animal prices but sell more slowly. Most serious breeders keep a mix across the size range to cover different demand.
One size doesn't fit all care guides
The only thing I'd stress is that a care guide written for Armadillidium vulgare doesn't automatically apply to Trichorhina tomentosa or Cubaris. Humidity requirements differ, temperature preferences vary, and breeding rates are very different. Dwarf species tend to be more tolerant of consistently damp conditions, while many Armadillidium prefer things drier than beginners expect. Always check species-specific guidance rather than assuming one approach covers everything.