There are loads of online Moxon dimension calculators, a quick Google search should find you one. The one I used was http://ab1jx.1apps.com/ham/calcs/moxon/index.html. They provide this diagram showing the Moxon antenna dimensions, the calculated values for the 20m WSPR frequency 14.0956MHz are below.

A: 7.7594 M

B: 1.1882 M

C: 0.1892 M

D: 1.4385 M

E: 2.8159 M

For my Moxon I intend to follow a well-trodden path of making a support in the form of a large X supporting each corner.