1 tile is ~32 x 32 km big in civ IV if I'm correct (its show 1000 square km).
It really depends on the Map size doesn't it

And sizes (and time) doesn't really make any sense in Civ. F.ex. as the turn step goes from 10 year/turn to 5 year/ turn, but the units move the same distance. Did they suddenly find a new and VERY efficient way of traveling, and its not even part of a scientific discovery
But regarding archers, could'nt they just stand on the edge of one tile and shoot into the neighbor tile?
OTOH what about rifles? Wouldn't they too needed to be ranged (in CIV V)?
TBH the worst (IMHO) about civ IV is the Suicide Sieges. THAT doesn't make any sense whatsoever.
The archers should have been some sort of a support troop instead of a direct attacker, and the sieges should (again IMHO) have been ranged.
The later I did in my mod, where catapults, trebs and bombards are range one (along with Frigates, Iron Frigates and a few other ships), cannons (and some ships) are range 2 and artillery, mobile artillery, battleships, missile artillery are range 3. Besides that a siege unit in a city (fortification) will get +1 in range. However, the sieges can return fire from a ranged attack, to level out the advantage.