You can obtain your first Lythian by completing the New User Tutorial.
Pre-designed Lythians from the Adoptables Marketplace, where official designers and other players can sell characters.
You can also get MYO Tickets to create more of your own Lythians from the Patreon Gem Shop, events, and sometimes in limited numbers from Truffle's Treasure Trove.
You can view a comprehensive chart of Lythian sizes via this page: Lythian Size Comparison
You can find more information regarding size on the Species Guide page of the individual species. However, all of these are just general guidelines and we will still allow designs outside the provided sizing parameters. We just ask that they be within reason! (So no four-inch Chuikha, for example.)