Use .addBox method instead.
Example code:
this.bipedBody = new ModelRenderer(this, 0, 1);
this.bipedBody.addBox(0F, 0F, 0F, 11, 11, 11);
this.bipedBody.setRotationPoint(-5F, -1F, -6F);
If you are using Eclipse you can view vanilla Minecraft entity models implementation
in "Project and External Dependencies/forge_1.15 (...) /net/minecraft/client/renderer/entity/model"
It could be helpful.