Regarding 'image size', there are really two issues:
1) # of pixels (horiz and vert) in the image. This parameter is what makes the image go beyond the actual screen size. HDTV screens are usually from 1280x720 ranging up to 1920x1080 pixels, so any 'pixel size' beyond these will be larger than the display device.
2) File Size (# of bytes in the image file on your computer). Generally if a software system tells you the image is too large, it's the file size the software has problems with. Some of the automobile enthusiast sites I go to limit 'file sizes' to 300 kilobytes in size.
If the image is NOT compressed, there is a direct correllation between the two parameters above, but for compressed images there can be large differences in the two. Most of us use JPEG compression (filename.jpg) for our image files to save space on our computer disks, and for these files you can use 'free software' which works very good. One such software that I use is called 'Light Image Resizer' which works very well and very quickly and can 'size' both of the parameters above. You can download it for free here.
https://download.cnet.com/Light-Imag...-10529104.html