yes, FFS will allow you to build a weapon of just about any size. Making a fusion gun with a single space-hex of range (30,000 km), however, would be impractical.
The laser design system seems to have a bit of a flaw in it, such that you can conceivably build gigantic bay or even spinal sized lasers, and these things absolutely rule space combat. Lasers have the wonderful property of having better and better armor-piercing capability the more powrful they are - the bigger they are, the tougher they are, out of proportion with the other weapons.
This is probably not a problem with physics, but does present a problem when it comes to making a game that has other weapons; there is simply no excuse to NOT build the largest laser you can, and forgo all others. No amount of armor and sandcasters can withstand a big enough laser.
FFS2 implemented a power limitation for lasers, to control this gaping hole in an otherwise excellent product. They suppose that the power limitation is TL-based, so that as your tech improves, you can make a more powerful laser, but even at TL 15, lasers are still basically limited to turret/barbette size.
At low TLs, like 10, you need a bay to mount a laser, and have little fear of running into the power limit, but higher tech (especially grav focusing) allows you to shrink the parts so they fit inside a turret, and still do plenty of damage.
The most important difference between FFS and the other design systems that I know of is that FFS gives you "real-world" values. That is, you know how much energy you are imparting on your target at what range, so if you want, you can convert that to another game system, or determine how many shots it takes to melt an icecap. The other systems simply give you a code, which you have to look up to see what it does in comparison to a certain code of armor, and you have little if any idea what the system could do in the real world, nevermind the absolute lack of customization.
