operating volume

The capacity inside the boiler is not related to the steam content of the system, as you've discovered it only takes a thimble worth of water turned into steam to fill all your pipes.

What causes low content boilers to go on low water level is the slowness of the return pipes. In some systems (like large horizontal homes with low sloped returns) there can be lots of condensate just loitering around in the returns just idling back to the boiler. On shut off it eventually all returns, but with an operating volume delay - thus the need for water capacity at the boiler.

You can get a feel for that situation by observing your operating boiler and taking note of how much the level falls on start up until it reaches steady state. In the summer, you can also test all this by pouring simulated condensate at the headwaters of the returns then watch the exit hole at the boiler. You'll get a very rough idea of your return operation.

As far as your boiler selection, ask the supplier what the differential capacity is between normal operating level, low level and high level. That number may be more useful.

I would tend to prefer the higher capacity, it is less problematic without a feed tank (which is overkill in a home set up) and it is less problematic for dissolved solids concentration and blow down needs.

Happy to see you steaming.

Don't forget, all new traps are always a good idea. Have them changed / replaced along with the boiler. Choose those with the best venting stats as per the venting chart by Gerry Gill and Steve Pajek, available here online, don't just choose the most easily available locally.

Are you located in a major steam country?