mac guns aren't necessarily firing humongous shells, though the smallest example i've heard of still fired shots the size of a door, a round of that size could still cause major destruction if fired at a planet like the earth.
depends on personal opinion and what size you think a ship is what size one makes the projectiles.
vidboi suggested large projectiles because hit detection with railguns in BSF is very finicky, not because of any preference for established real-world or sci-fi concepts regarding magnetically accelerated kinetic weaponry.
Tiny railgun projectiles going at excess of 45 speeds will skip several sections before triggering another hit, which makes the weapon useless as it would most likely pass through the target entirely while dealing less than optimum damage. For example, we had a problem in the metagame where even speed 36 railguns of default projectile size would score 3~4 of their potential 10 hits while it travelled at a 45 degree angle across destroyer-sized ships. However, dead-on hits from the front where the projectile would slam through the entire length of the ship would score 9 hits at best.
In addition, I suggest using custom bullet sprites for sized up railgun projectiles as the default graphic can look pretty ugly as it stretches like a rubber balloon across the map.
samkiller911, vidboi: If you'd like, I can make available one of my own railgun projectiles that was slated for use with a metagame ship.