I would suggest that the sector range be at least three times the maximum range from base that you want your ships to operate at - you'll use one third of the fuel getting there, another third going back, and the last third will allow you some spare fuel for combat during the fleet's time on station. Note that if sum(weapon energy per second, cruise energy, shield regeneration) is about equal to hyperdrive energy, your maximum combat time on station is roughly equal to the time that it takes your fleet to get there, unless you're willing to suffer a slow crawl to the nearest refueling point. Note that this is a minimum design range, not a maximum or even an average, and that fleets with proportionally more energy use outside of hyperdrive should have proportionally larger percentages of their range dedicated to on station use rather than in transit use (i.e. if your hyperdrive energy use is ~50% of what the weapons + shield regeneration + cruise requires, then I would suggest that the minimum range should be ~4 times the maximum distance from base that you want the ships to operate at, which reserves ~25% of the fuel for the trip to the station, another 25% for the trip back, and 50% for time on station, which still only means you have roughly the same amount of time for combat on station as you require for moving the ship to or from the station).
A second minimum range suggestion is that you probably want to at least be able to send a fleet from the central portion of your empire to the edge of your empire, even if it won't have much fuel for combat at the end of the trip; better would be if your ships had sufficient range to cross the longest dimension of your empire without running completely out of fuel. This makes it easier to reposition fleets within your empire as you do not need to worry about stopping halfway to hit a refueling point or suffer severely reduced speed for the rest of the trip. Then add a few extra fuel cells in to give you a bit of a margin for error (e.g. you didn't stop to top off the tanks before ordering Death Squadron to run to the other side of the empire) or a bit of combat time at the destination.
A third suggestion is that you probably want to have at least enough fuel for ~1 minute of combat time. ~1 minute of combat time translates to 60*(weapon energy per second + shield recharge + cruise energy use + static energy use) energy. Energy translates to a fuel capacity requirement through the reactor efficiency, which I believe is given in terms of fuel units per unit of energy. Caslon reactors are less efficient than hydrogen reactors, so a caslon design will require more fuel than a hydrogen design, but on the other hand the caslon reactors are generally more powerful than the equivalent hydrogen reactors (as a very rough estimate, an early caslon design will require about 4 times as much fuel as the energy requirement given by the formula above, while an early hydrogen design will require about 3 times as much fuel as the energy given by the formula).
I would say that a good number for an early-game Gerax Hyperdrive size 200-300 vessel is somewhere around 10 or so sectors of range; having a bit more range is better, while less probably won't kill the design.