In dimensios

I run easel on my X-carve 1000x1000 it won’t let me change any dimensions I have already put in IE: in material on the dimensions I had a 3 ,but that was to big ,so I wanted to change it to a 2 and it wont let me and it on it only gives me 2 choices 20 & 24, why ? it usta let me change it.