Merge pull request MicrosoftDocs#3420 from get-itips/patch-39

Added recommended size for previewImageUrl

Author: trent-green

sharepoint/sharepoint-ps/sharepoint-online/Add-SPOSiteDesign.md

@@ -111,7 +111,7 @@ Accept wildcard characters: False
 ```
 
 ### -PreviewImageUrl
-The URL of a preview image. If none is specified, SharePoint uses a generic image.
+The URL of a preview image. If none is specified, SharePoint uses a generic image. Recommended size is 400 x 421 pixels.
 
 ```yaml
 Type: String