given the average traffic per user you can deduce the traffic of a region area having the total number of users.
knowing the capacity (traffic) that can be supported by a site you can compute the number of site needed. you will also need to have the area of the region to compute the area covered by a site. once you have this input you can deduce the radius with the formula
S_cell = (3 * square(R) * squareroot(3))/2
this way of computing the cell radius is theorical.
the typical method is what has been explained by pix below remembering that the
Maximum Allowed Path Loss=min(PathLoss DL, PathLoss UL) to avoid unbalanced link budget