This I understand it can happen later in the game also considering the increasing in capability of both production and shipyards, however I would like to ask for your opinion of what I currently do.
So as you know we cannot expand the displacement capability of the shipyards along with the slipways so we either do one or the other.
I personally set my ship sizes by tonnage displacement (so let's say that a DD will go from 10,000t to 14,500t) and once I reach that tonnage I just increase the slipways.
I get to the point: is it worth to have shipyards keeping increasing tonnage and design bigger ships (as be suggested by few users already) when you could simply increase your output?
For instance when you have your shipyard ready to to get 5 or 6 18k DD I may have already the capability of producing 18 at the time.
For a simple answer let's assume the tech is the same for the both of us and role-play is not in the picture.
One concern with building lots of slipways is retooling. You cannot retool while you are building a new slipway; nor (AFAICT) can you partially build a new slipway, retool, then finish the slipway. Since new slipways can take a REALLY long time, I often find myself increasing my production capacity by increasing size rather than adding slipways, simply so I can retool if I need to. But I tend to make lots and lots of versions of very similar ships as tech improvements roll out, so I retool my shipyards more often than strictly optimal.
For example, my last game that got anywhere had Hezrou class light cruisers, flights 1 through 5. Same size, same main battery, but incremental electronic, propulsion, and defensive upgrades. I also had 4 flights of Fate class missile cruisers that I was building at the same time, and my needs for 1 or the other swung wildly so I found myself needing to retool my shipyards from Hezrous to Fates and back again. It would have taken ~2 years to add another slipway, during which time who knows what would happen?
mmmm interesting point. Probably as I retool only for very specific ships and produce a new class for each new generation I may not be impacted much by that. Add this to the 10% research rate and you can see how much this won't really be a big issue. But I realize this may apply only to me or to the people which share a similar way to play to mine.
My generations are generally determined by the Engine Tech. Missiles are mostly the same sizes so launchers and magazines usually last for very long.
So my Rampant Corvette design (It's a Laser Beam ship) will be pretty much the same till I discover at least Ion engines. I start with Nuclear Pulse tech. When that happens I already have more powerful lasers etc so the Rampant just goes obsolete and I start producing a new class corvette with all the latest designs, while my Independent Corvette Leader design (the eyes of the fleet) gets retooled as soon as new active sensors are available and get obsolete when the new gen is possible.
In a fleet where you have 9 of the Rampant to go with 1 Independent, retooling of 3 or 4 ships out of 50 from time to time it's not a biggie.
For maintenance purpose, every 2 gen, ships goes to scrapyard as I can easily produce 15 or 16 ships in a raw by that point, sometimes even more.
Again this is very simple talks as there is way more to it, such as how many minerals it will cots me and the benefit on the long run; but this is why we have excel installed, right?!