Ability to set custom size for device preview

[spydon]

Just like with the device preview, but having the possibility to set any size instead of the predefined ones.

[spydon]

@ogustavobelo this one you could take on if you'd want to. :)

[ogustavobelo]

@ogustavobelo this one you could take on if you'd want to. :)

Nice! Let me try.