How can I shorten "Add fractional dimension" and still keep its meaning clear?

I am working on a website that sells custom made blinds. Any blind can be made in widths and lengths ranging from 10 in 1/8th inch increments inches to about 100 – 7/8ths inches. If I just made a width drop-down, the selection list would be 720 rows long, which is utterly unusable, of course. So what I have done is supplied a text input for width where the user can enter any number for 10 to 100 (JavaScript verifies that numbers are in range and have no special characters) and then a drop-down where the user can select a width adder if needed, Selections are “None, 1/8, ¼, 3/8, ½, 5/8, ¾, 7/8”. Length, of course, is handled the same way.

My challenge is what to call that fractional adder drop-down so it is clear to users what it is there for. I don’t have room to write out “Add fractional dimension.” How can I get the idea across in half as much space as that takes?