at least 13/16th", sufficient to cut thru 3/4" stock.
I used a drill press. Larger holes requires a better, firmer, way to secure the material for cutting. Also the drill press provides a straighter boring tool at a 90 degree angle. I think it would be more difficult using this tool at an angle of entry.
Dear Customer, Thank you for your question. Please refer to the product features detailed on homedepot.com - 'Cuts holes ranging from 1 in. to 6 in. Diameter'. Sincerely, MIBRO Customer Care
Dear Customer, Thank you for your interest. Yes, a square ¼” cutter bar should work perfectly. Sincerely, MIBRO Customer Care