Rather small compared to the deep sea ones you are probably thinking of. They’re terrestrial so many people will call them rolly polys or something like that.
The largest species I currently have are Porcellio hoffmannseggi, as far as I know the largest terrestrial ones and they get to about 5 cm total length and maybe 3 cm body length.
I once tried to breed amphibious Ligia oceanica ones that I collected, commonly referred to as sea slaters. They were a bit bigger in terms of body length but needed saltwater and also didn’t fare well with room temperature I am afraid.
However the P. hofmannseggi are fairly easy to keep and inexpensive. They just need a small container with a bit of soil, leaf litter and some dead or rotting hardwood to hide under, I spray them about once per week. It’s a great hobby.
There are also small freshwater species that can be kept in aquarium but they are not sold anywhere and you can usually collect them yourself.