Its a feature that is missing from Chrome’s implementation of --app. I think that Chrome doesn’t have that feature anywhere – it doesn’t remember sizes of browser windows for me. But then again I prefer browser windows (and most application windows) as full screen anyway, or tiled.

One thing that you can do, if you live on a desktop with window rules – like KDE Plasma – is to set a window rule for the application to always open in a specific geometry.