I want to create the biggest circle I can that touches the east, south (the
street), and west (the driveway) sides.

Permit me to suggest that's not what you should do. Your raised bed
should be of such size and shape that you NEVER have to step on it.
In other words, you should be able to reach the center of your raised
bed while your feet are on the ground outside the raised bed.

vince norris