Gutter size is a drainage math problem, not a style choice. Multiply roof drainage area by the local 1-hour rainfall intensity (Toronto: ~40 mm/h). A 5" K-style aluminum gutter handles about 5,500 sq ft of roof; a 6" handles ~7,960. Any long steep roof plane over 1,000 sq ft or any valley dumping into a gutter is an automatic step-up to 6".
Downspouts: 2"×3" moves ~600 sq ft; 3"×4" moves ~1,200 sq ft. In Barrie we default to 3"×4" — the cost delta is under $80 per drop and it eliminates 90% of overflow callbacks.