Standard and Obake Anthuriums are sized by their widest width, typically found at the level of the spadix or nose.  The grey line in the image gives the typical location for sizing. Sizes are as follows:

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Exhibition: 7.5 inches and greater

Super large: 6.5 inches

Extra large: 5.5 inches

Large: 4.75 inches

Medium: 4 inches

Small: 3 inches

Designer: 2.5 inches and smaller