So happy you like it so far!
It's indeed not very easy to find (preparing for UI overhaul soon). It has to be enabled in Properties - Camera - Armory Renderer Props - Soft Shadows.
Once that is enabled, increasing lamp size should produce softer shadows. It works just for spot lamps currently (plus maybe sun), point lamps are now using cubemaps to cast shadows in all directions and soft shadows are not yet in. As you say it's quite tricky to get it right, going too big will produce noise.. (and costs performance). Increasing bias can help, but it's very annoying to tweak that. Will implement some other shadowing methods eventually, not based on shadowmaps.
I added a minimal example to the repository: