Go to Bourbaki, Groupes et Algebres de Lie, Ch.4-6 where the fundamental weights $\varpi_1, \ldots , \varpi_n$ of $X_n$ are listed. The vertices of the simplex are $$0 \ \mbox{ and } \ x_k \varpi_k, \ k=1,\ldots,n$$ where $x_k>0$ is such that $x_k \varpi_k$ lies on the fixed hyperplane of the last Cpxeter generator of the affine Weyl group. This hyperplane is given by the equation $$ \langle z , \alpha_0^{\vee}\rangle =0 $$ where $\alpha_0^\vee$ is highest coroot. The highest roots are also listed in the same book. Don't forget to swap $B_n \leftrightarrow C_n$, when looking for the highest coroot, because the coroot system is Langlands dual to the root system.

No, I don't know where this calculation has been carried. Please, compute the length of all the edges of this simplex yourself, and put it here for the benefit of the whole humankind:-))

Go to Bourbaki, Groupes et Algebres de Lie, Ch.4-6 where the fundamental weights $\varpi_1, \ldots , \varpi_n$ of $X_n$ are listed. The vertices of the simplex are $$0 \ \mbox{ and } \ x_k \varpi_k, \ k=1,\ldots,n$$ where $x_k>0$ is such that $x_k \varpi_k$ lies on the fixed hyperplane of the last Coxeter generator of the affine Weyl group. This hyperplane is given by the equation $$ \langle z , \alpha_0^{\vee}\rangle =1 $$ where $\alpha_0^\vee$ is highest coroot. The highest roots are also listed in the same book. Don't forget to swap $B_n \leftrightarrow C_n$, when looking for the highest coroot, because the coroot system is Langlands dual to the root system.

No, I don't know where this calculation has been carried out. Please, compute the length of all the edges of this simplex yourself, and put it here for the benefit of the whole humankind:-))