Include an optional screen size in the WindowsDesktop spec

api/proto/teleport/legacy/types/types.proto

@@ -4995,6 +4995,15 @@ message WindowsDesktopSpecV3 {
   // NonAD marks this desktop as a standalone host that is
   // not joined to an Active Directory domain.
   bool NonAD = 4 [(gogoproto.jsontag) = "non_ad"];
+  // ScreenSize specifies the size of the screen to use for sessions
+  // on this host. In most cases this should be unspecified, in which
+  // case Teleport will fill the browser window.
+  Resolution ScreenSize = 5 [(gogoproto.jsontag) = "screen_size,omitempty"];
+}
+
+message Resolution {
+  uint32 Width = 1 [(gogoproto.jsontag) = "width,omitempty"];
+  uint32 Height = 2 [(gogoproto.jsontag) = "height,omitempty"];
 }
 
 // RegisterUsingTokenRequest is a request to register with the auth server using

api/types/desktop.go

@@ -25,6 +25,11 @@ import (
 	"github.com/gravitational/teleport/api/utils"
 )
 
+const (
+	MaxRDPScreenWidth  = 8192
+	MaxRDPScreenHeight = 8192
+)
+
 // WindowsDesktopService represents a Windows desktop service instance.
 type WindowsDesktopService interface {
 	// ResourceWithLabels provides common resource methods.
@@ -132,6 +137,10 @@ type WindowsDesktop interface {
 	// NonAD checks whether this is a standalone host that
 	// is not joined to an Active Directory domain.
 	NonAD() bool
+	// GetScreenSize returns the desired size of the screen to use for sessions
+	// to this host. Returns (0, 0) if no screen size is set, which means to
+	// use the size passed by the client over TDP.
+	GetScreenSize() (width, height uint32)
 	// Copy returns a copy of this windows desktop
 	Copy() *WindowsDesktopV3
 	// CloneResource returns a copy of the WindowDesktop as a ResourceWithLabels
@@ -179,9 +188,24 @@ func (d *WindowsDesktopV3) CheckAndSetDefaults() error {
 	if err := d.ResourceHeader.CheckAndSetDefaults(); err != nil {
 		return trace.Wrap(err)
 	}
+
+	if d.Spec.ScreenSize != nil {
+		if d.Spec.ScreenSize.Width > MaxRDPScreenWidth || d.Spec.ScreenSize.Height > MaxRDPScreenHeight {
+			return trace.BadParameter("invalid screen size %dx%d (maximum %dx%d)",
+				d.Spec.ScreenSize.Width, d.Spec.ScreenSize.Height, MaxRDPScreenWidth, MaxRDPScreenHeight)
+		}
+	}
+
 	return nil
 }
 
+func (d *WindowsDesktopV3) GetScreenSize() (width, height uint32) {
+	if d.Spec.ScreenSize == nil {
+		return 0, 0
+	}
+	return d.Spec.ScreenSize.Width, d.Spec.ScreenSize.Height
+}
+
 // NonAD checks whether host is part of Active Directory
 func (d *WindowsDesktopV3) NonAD() bool {
 	return d.Spec.NonAD