freegodsoul
I didn’t find any engine parameter (Application::engineParameters_) responding for maximization/minimization of window,
so I’ve put this code in begining of my Application::Start() implementation.
Graphics* gfx = GetSubsystem<Graphics>();
GraphicsImpl* gfximpl = gfx->GetImpl();
SDL_Window* sdlwnd = gfximpl->GetWindow();
SDL_MaximizeWindow( sdlwnd );
P.S. Out-of-box Editor app already has maximized window, so I’m not sure if this approach is non-redundant. It looks like workaround, but it’s working. 
[EDIT]
After some research I’ve found the method Graphics::Maximize(). Final version:
Graphics* gfx = GetSubsystem<Graphics>();
gfx->Maximize();