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As far as I know there is no way to set that default size in the UI, however this isn't really a customization question, as it doesn't involve using the API. You may have more luck asking this kind of questions in the general section of the forum. A product specialist might be able to answer you.
If you are interested in rendering an image through the API, below is a sample. You can control the size of the image using RenderManager properties:
Public Sub StudioRender() ' Find the Studio Add-In Dim oStudioAddin As ApplicationAddIn Set oStudioAddin = ThisApplication.ApplicationAddIns.ItemById("{F3D38928-74D1-4814-8C24-A74CE8F3B2E3}") If oStudioAddin Is Nothing Then MsgBox "Inventor Studio Addin not loaded." Exit Sub End If ' Get the Inventor Studio object. Dim oInventorStudio As InventorStudioLib.InventorStudio Set oInventorStudio = oStudioAddin.Automation ' Get the environment manager for the active document. Dim oEnvironmentMgr As EnvironmentManager Set oEnvironmentMgr = ThisApplication.ActiveDocument.EnvironmentManager ' Get the current environment. Dim oCurrentEnvironment As Environment Dim strEditID As String Call oEnvironmentMgr.GetCurrentEnvironment(oCurrentEnvironment, strEditID) ' Activate the Studio environment. Dim oStudioEnvironment As Environment Set oStudioEnvironment = oInventorStudio.activate ' Get a reference to the RenderManager Dim oRenderManager As InventorStudioLib.RenderManager Set oRenderManager = oInventorStudio.RenderManager ' Set the active lighting style. oInventorStudio.ActiveLightingStyle = oInventorStudio.StudioLightingStyles.item("Pleasant") ' Set the Camera. oRenderManager.camera = oInventorStudio.StudioCameras.item("MyCamera") ' Set the active Scene. oInventorStudio.ActiveSceneStyle = oInventorStudio.StudioSceneStyles.item("Starfield") ' Set the output resolution to the size of the current view. oRenderManager.ImageWidth = ThisApplication.ActiveView.width oRenderManager.ImageHeight = ThisApplication.ActiveView.height 'oRenderManager.AntiAliasing ' Render the image. Call oRenderManager.RenderToFile("C:\Temp\Render.bmp") ' Reset the environment to whatever it was before ' we entered the Studio environment. Call oEnvironmentMgr.SetCurrentEnvironment(oCurrentEnvironment) End Sub