It's a bit of a hack, but try designing ships with known masses, and multiples of that base mass and a fleet speed. I try to design around 5000 ton increments, for instance. It's hard to stick to in the early game, but then early game stuff tends to be what you've got, not what you want anyway. Typically, I have a 2500 ton class (Corvettes, typically just weapon platforms, designed to add firepower to FF squadrons), a 5000 ton class (Frigates (FF), much the same firepower as a Corvette, but with improved fuel capacity, the sensors the squadron needs, some armour, maybe some spare crew berths to pick up survivors), a 10,000 ton class (Destroyer (DD), first specialised class. Some design will be anti-missile / fighter / FAC, some anti-ship, maybe a few with honking great sensors and not much else, maybe a very small carrier design even. A DDCV isn't going to be part of your expeditionary fleets, but if you only give them Corvette engines, they make great defensive units over fixed points like colonies or JPs), and so on.
Then you only need to design an engine "tuned" to a 5k ton ship, and pile them on as appropriate for a consistent fleet speed across your entire force. You need a few smaller ones for Corvettes (I often have dedicated PD Corvettes have a slightly higher max speed so they can reorient in a fleet moving at max), fighters and FACs as appropriate for their missions (and that will change a lot according to current circumstances)
