Lorna; depends on the species (which can be quite hard to tell apart) but from 2" to 4".
They are easy to look after once you have a tank set up well for them, but getting a suitable tank ready requires;
1) more money than usual (extra filters or 1 xL filter, airstones)
2) patience (get a decent algae build up before adding fish, make sure tank is well and truly cycled)
3) effort (extra water changes, more water tests)
4) limitations (other species that can be kept with them, plants that are suitable for a cold, rivertank set-up)
Theres a fair bit more to it, but those are the basic things to bear in mind before purchasing.
But its well worth it, they are gorgeous; all cute and wiggly, very interesting to keep. Brilliant IMNSHO!!
Make a thread if you want on oddballs or cyprinids and i'm sure you'll get a fair bit of advice or search under beaufortia, hillstream, gastromyzon and pseudogastromyzon on the Loaches Online website for info....
Sorry to hijack
Peas!