Here's a trick to remember whether to use lie or lay: When they get up again, do you use rise or raise?
I lie down, I rise up.
I lay down the book, I raise up the book.
The book is lying on the table, the book is rising off the table.
Hermione lays the feather on her desk, Hermione raises the feather off her desk.
Jaskier lies next to Geralt in the tiny bed, Jaskier rises from the enormous bed when the very sexy but insane witch comes after him.
Geralt lays Jaskier's unconscious body on the enormous bed, Geralt raises Jaskier up onto Roach's back.
Jaskier's length lies... well, you get the picture. If the reverse is rise, use lie. If the reverse is raise use lay.
The confusion comes from the past tense: The past tense of lie is lay (Jaskier climbed the stairs and then lay on the tiny bed), and the past tense of lay is laid (Geralt carried Jaskier up the stairs and laid him on the enormous bed).