When getting started on a design, you often first need to first create mockups and wireframes. Tools commonly used by designers for prototyping include Sketch and Photoshop, both of which integrate with InVision through Craft.
Image calculators like this are helpful for validating image sizes before uploading and testing on your iOS device. You can also check out material.io for popular image sizing, including desktop and laptop monitors. For more information on screen sizing, be sure to check out this article: What size should my screens be?
You will need to edit your images using an external image editor (InVision Studio, Sketch, Adobe Photoshop, Pencil Project, etc.) prior to uploading your image files in InVision.