Summarize:

Use the USB cable that came with the device. The process varies a little depending on whether you're using Windows or Mac:   Windows - Double-click "My Computer", then double-click the iOS device in the "Devices and Drives" section.  Mac - Double-click the iOS device icon that appears on your desktop.   Once you've found the image, you can open a new window displaying information about it.   Windows - Right-click on the image, then click Properties.  Mac - Select the image, hold down Command, and tap I. You should be able to see the easy-to-read size (e.g. 1.67 MB) as well as the actual exact size (e.g. 1,671,780 bytes). The photo's size should be next to a heading that says "Size" or "File Size."
Connect your iOS device to your computer. Open your iOS device on your computer. Double-click the "DCIM" folder. Find the image that you want to check. Open the details for the image file. Review the photo's size.