Configuring a default display resolution on startup

Just started using QEMU/KVM on Tumbleweed, and seems to work OK, but I am a little confused, coming from using VirtualBox for a long time...

Initial question: How do I get the guest to startup at the correct resolution/size?

Using a Win10 guest, and have the drivers installed (I think?), but even though the resolution of the guest is set to 1680x1050, it never starts that size, and I have to rescale it??

There just doesn't seem to be any way to configure the guest window??

Thanks.