A "cedar tree" may refer to the true cedars of the Old World -- like the famous Cedars of Lebanon, frequently referenced in biblical texts -- or to other conifers, especially various members of the cypress family. Some of these trees -- most notably the western red-cedar of the Pacific Northwest, one of those "false" cedars -- can grow to impressive diameters.
All three species of true cedar grow to be relatively stout. The Gymnosperm Database reports that deodar, Atlas and Lebanon cedars all may be close to 11 feet in diameter measured at breast height. These conifers grow in the Himalayas and highlands of the Mediterranean basin and are widely planted as ornamentals the world over.
The western red-cedar is a true giant, often growing massively larger than any of the true cedars in the lush temperate rain forests of the US and British Columbia it calls home. The biggest known specimens -- on Washington's Olympic Peninsula and Vancouver Island -- approach 20 feet across.
The yellow- or Alaska-cedar, which shares its range in the Pacific Northwest with the bigger western red-cedar, may nonetheless exceed 13 feet in diameter as exceptional specimens. The northern white-cedar of eastern North America, commonly termed "American arborvitae" in the horticultural trade, is known to reach over 5 feet in diameter.