Guns that can shoot miles away are actually more properly treated as Launchers, since at that point we're talking about artillery cannons. (With the exception of the Death Star gun, whose interstellar range is a special case.)
The balance issue isn't that a size 5 gun shoots so far, it's that it shoots so far compared to a size 1 gun. It should be easy for the size 5 to start wiping out lesser opposition before it comes in range to fire back, but not that easy.
The problem, now that I've scaled back Use ratings, is that as guns get bigger, they now get Size times more expensive (Cost-wise), but Size-squared times more effective (Damage and Range-wise). And that won't do! So here are the next steps.
Guns: Range changed to Size + 5". This nerfs the range of big guns to where they still have an advantage but it's not overpowering. The advantage of bigger Guns is primarily in increased damage.
Launchers: Range changed to Size x 6", removing the unnecessary die roll; Use changed to WS x 2, because it's funnier to miss with Launchers than with any other weapon type, and so I want to make it more likely. Damage as determined by payload: either Explosive Damage for Explosives, or Collision Damage for regular objects (i.e., payload-Size times 1d6). Maximum payload size 1/2 Launcher Size, rounded up. Launchers take over the long-range role, doing less damage than Guns but striking much further away. (I may need to add a Ballistix subsection under the Heavy Explosives subsection in order to clarify all this.)
I really want to work in the idea of ballistik delay, though I haven't yet decided where to make the cutoff: When you launch a payload into the air, you plant down a little target marker at the spot you're aiming for. But you don't make the Attack Roll until the beginning of your next turn, because it takes a while to fly through the air, and until it comes down again, you don't know whether it hit the target or how much it missed by. All you hear is that awesome whistling sound of impending death from above, giving troops time to panic and scatter. It could probably be a simple distance cutoff - for instance, any shots past 10" would be subject to flight delays.