Rockler Catalog is selling very convenient jig online that does circles up to 52"
Round cut holes can be easily made using router jig method, often sold as accessory.
A Jig can be made from the piece of 1/2" x 48" x 8" plywood. Router is installed on one end of the jig using locally obtained longer base screws.
Remove the plasic plate on the bottom of your router and mark mounting holes and center hole. Simply use the router's removable baseplate as a template. The center hole about 3/4". Bring the base plate mounting screws to your local hardware store and procur ones that are 1/2" longer to accommodate the thickness of your plywood template (1/2").
Predrill hole on another end with the distance to the cutting edge of the bit equal to the required hole radius. Drive a screw through the hole to fix it in the center of the circle in the floor. Verify again to make sure that the size is set correctly.
Now you can turn on the router, lower the bit and rotate the jig you made around the pivot, cutting the circle. Cut the circle in several passes of 1/8" to 1/4" deep. For inside cutting, move the router clockwise.