I went searching for a knitted D20 pattern online and found just two, a cat toy sized one on here(Ravelry) and a HUGE one on Crafster.org
The Crafster pattern was a starting point but I made some improvements to it.

I knitted the triangles together where possible instead of making 20 separate ones and then sewing them all together, I made a number chart and I used solid foam as the fill instead of polly-fill, so it retained a true polyhedral shape, an Icosahedron to be exact.

The size/gauge isn’t important, I’ve found that no matter the size of yarn so long as you use the correct sized needles the triangles turn out perfect.

I used 1/2 to 2/3 of a Caron Natural One Pound skein for my big D20. Knitted on size 8 needles.