Server Configuration – Serving Two Different Subnets with One Server

Well, sometimes you find some consumer hardware with that mask hard coded here and there. But that's relatively rare and not a deciding factor. Also you want to stick within the predefined private ip ranges, that's the bigger consideration.

Usually common practice for choosing defaults (in my neck of the woods at least) is to use the tightest mask possible and that one is an easy one for humans to deal with even if it isn't the absolute tightest.

You can use whatever you want though just be careful to not open up a range that starts to conflict with internet IPs. There's a list of reserved addresses at https://en.m.wikipedia.org/wiki/Reserved_IP_addresses. E.g. 192.x.x.x/255.0.0.0 is not acceptable for private LAN use.

I know this doesn't answer your question fully. Barring other answers I'll do some research on more concrete reasons why the default is /24 when I'm not on my phone. I strongly suspect it's either tradition, or the safest bet given that unwise users may choose nonprivate ranges.

My first problem is, how can I configure two (or more) ethernet interfaces to the same network, like at the home routers has.

Routers route between networks, so each router interface needs to be in a different network. Don't be confused by what you think is a home router. The home router has separate networks on the router interfaces, but you are probably meaning a switch module in a home router. You need a switch or a switch module in a router for that, but those are not router interfaces.

Also how can I set the ethernet interface is WAN connection not LAN connection? Because I want to connect routers together.

For a router, there really is no real distinction between WAN and LAN. Some interfaces, e.g. serial interfaces, are more likely to be used for WAN connections, while some router interfaces, e.g. ethernet, are more likely to be used for LAN connections, but you could use just about any type of router interface for either connection type. There are no commands that say, "This is a WAN interface." You can connect two routers together with like interfaces.