Dismissing eidolon is not a command but reversal of ritual that summoned it. It would be probably analogous (and using the same action) to dismissal of existing spell which has to be done in range of the spell. So the range of dismissal will be probably equal to Life Link (10,000 feet unless extended with other spells and effects). However, it is irrelevant because beyond that range eidolon is immediately sent home plane, thus it is always either "in range" of dismissal or it is already dismisssed.