Custom Oval Stone Size In Gem Studio

Hello,

Can anyone help me with getting a custom size for an oval shapped stone. Im using the GemStudio button and when I enter two numbers for the size, I just get two stones.  I need my stone to be 5.1mm x 4.2mm. The standard size I am getting when I enter 5.1 as the number is 5.1mm x 3.17mm (or something close to that). Which is nowhere near what I need.

Any help will be greatly appreciated

Thank you!