Here is the dimension that widely used for supporting plants
—A. Diameter from 3.0mm-5.5mm, length from 300mm-600mm, which are used for supporting flowers, especially in the Netherlands, which is famous for flowers.
—B. Diameter from 6.0mm-8.0mm, length alway be 4”(=1220mm), 1500mm, 6”(=1830mm), which are widely used for supporting grapes, peppers and small trees, etc. 
—C. Diameter from 8.0mm-12.0mm, length alway be 1500mm, 2000mm-2400mm, which are widely used for supporting tomatoes, peppers, and vine, etc.
—D. OD*ID: 25*21mm, 28*24mm, 32*28mm, 35*30mm, 41*37mm, length 3000mm-3500mm, which are widely used for supporting trees, like apple trees.
—E. Diameter 15mm and 16mm, and the flat bar are widely used in plant, too.
—F. For the small diameter, the customer will require that one side should be taper, just like a pencil point.
—By the way, we can send you some samples to do the testing for free if it is available.