Here you see all of our large tool rests, from the left- large rest with six inch stem then large with 7 inch stem and large with 8 inch stem, behind that one there is an extra large with 8 inch stem. The XL is also available with 7 inch stem, The large rest top bar is 1” diameter and is 14 inches long measured along the curve of the rest and projects 3 inches out on the short end and 3 inches past the bend. The extra large top bar is the same but projects 4 inches at both ends and is 16 inches overall. The stem diameter on all these is 1 inch and the overall height is measured from the top of the bar to the bottom of the stem.
To know what height you need simply measure your existing tool rest that came with your lathe.