First thing off the bat: For a beam ship, that is pathetically slow. I can't tell from the design what tech engines you're using, but in any case engines should generally be a good percentage of the ship's total size; even more so with beam ships, which have to close really fast to get into firing range without being blown up by missiles first. If you can tell me what the engine design is, I can give you pointers on basic engine design(though one general tip is that engine power for warships is typically 1.25 to 1.5x power for standard warsips, with higher power mod for short range ships).
Next, that is a LOT of fuel for a warship, which typically spends most of its time parked orbiting fleet bases and not moving. Let me put it this way; in 6.0, a warship with 100 days fuel is considered a long rage ship for extended deployment. You'll typically have a fleet train with huge tankers behind your fleet.