Most undersized banks are undersized because the calculation started with a battery, not with a boat. The right order is the other way round: work out what the boat consumes in a typical day, decide how long you want to go between charges, then choose chemistry and capacity to suit.
The second half of the job, and the one people skip, is making sure you can actually put that energy back in.
List every load, its current draw and the hours it genuinely runs. Fridges dominate on most boats, and they run far longer in August than the spec sheet suggests. Autopilots on a windy passage are the other big surprise.
Lead acid banks should not be routinely taken far down without shortening their life, so a large part of the rated capacity is not really available. Lithium can use much more of its rated capacity, which is why a smaller lithium bank often replaces a much larger lead acid one.
Compare banks on usable amp hours and on weight and space, not on the headline figure.
A big bank with a small charge source simply stays flat. Increasing capacity usually means looking again at alternator output and regulation, mains charger size and solar, plus cable sizing and fusing for the higher currents involved.
This is the part that turns a battery purchase into a system change, and it is where most disappointing upgrades go wrong.
Batteries must be secured against a knockdown, ventilated appropriately for the chemistry, and fused close to the terminals. Weight distribution matters on smaller boats. All of this constrains the practical answer as much as the arithmetic does.
There is no universal figure. A weekend coastal boat with a fridge and lights has very different needs from a liveaboard with an inverter and heating, which is why we start from your actual usage.
Mixing ages, and especially mixing chemistries, is generally a bad idea. The weakest battery sets the behaviour of the bank and you lose the benefit of the new one.
Only if the charging can fill it. If the real problem is charge input or a parasitic drain, a bigger bank just takes longer to disappoint you.
We size banks around real usage, check the charging can keep up, and quote the whole system in writing.