Originally Posted by Jan in VA

Draw it out on graph paper.
I made a 6.5" square.
I drew the inner logs at 1" (because the seams took up the .5")
I drew the four outer logs at 1.25" (because only one seam allowance has been used at this point)
I drew the hearth 'block' at 2.5" because that was the only size left in order for the block to turn out at 6.5"

This gives you a log cabin with two logs on each side of a large hearth.

ANOTHER OPTION:
Make your hearth block a 4-patch of 1.5" squares and make it 3 squares of dark with 1 light (or 3 light and 1 dark) logs.
This will make your log cabin appear not to have a hearth block, so if that hearth color isn't necessary for the design, this is a good way to go.

Jan in VA