the size of the layer doesnt matter - when ye set the x and y speed to 0 (and check 'tile') the displayed tiles will be all four corners of the layer
since it doesnt matter its of course easiest to make the size 20x15

the bottom left corner (some 5 or 6 tiles big in height, and 16 to 17 tiles in width) of the layer in jcs, will in the game be placed at the top of the screen, moved 5 or 6 tiles to the right

so what ye put in the upper left corner will be placed right under that

ehhh... just try it - ye'll figure it out