Analog, as Nazul says, setup your document say using the 16:10 aspect ratio and just imputing any one either the horizontal or the vertical the other should change accordingly to the proper dimension as long as you have “constrained proportions” checked on. The screen should snap to that size when you apply the check mark to accept changes and out from the document setup. Else go to edit menu: Preferences and make sure you set it up as shown in screengrab.

Set your preview display to camera view. Go “Create” and plop a primitive onto your scene. If it cannot be seen or is way off then you need to configure your camera for this new document size and save this as your Default start up file. Now I’m working off Bryce 5.1 and I’m hoping they haven’t changed this since. First, press 2 on the keyboard to bring you to the top view and select (highlight) the camera. Now position the camera with x = 0 in the camera attributes “A” origin. This centers the camera along the x axis of the coordinate system. Press 3 on the keyboard to bring you to the right viewport view and move the camera using the edit tool up or down as you need it to center the primitive. Go back to the top view and pull the camera in or out along the z axis. When you’re done and your primitive is centered on screen as you like it click on the camera’s attribute (A) button. Where it says “origin” make sure x = 0 (centered camera) and these are your new camera settings. I start out by having my director camera same as my camera view (click on Director to camera in the control camera options to do this). Then you can move in director mode to find a more pleasing POV. Then hit Camera to Director to place the camera where you want it.

Delete the primitive and save this file in the application directory as your Default.Br## whatever number version Bryce you’re using. Shut down Bryce and restart it. Bryce should now start at the new settings resolution dimensions and centered camera. Now when you create a new primitive it should be where you selected it to appear. Also, because you’re starting from the center of the x axis moving your mouse to the left will actually move you in the negative along the x axis and moving to the right positive on the axis. In the screengrab above you can see in my camera attribute dropdown that I’m centered on the x axis = 0, 3 Brycean unites above the ground and -150.50 unites back along the z axis. My screen resolution is 1600 x 1200 which is 4:3 aspect ratio. I hope this answers your question.