Whether your proposed graphic meets the guidelines should be apparent if you look at the properties of your graphic. I haven't seen any examples of graphics that don't show both the file size AND the height/width. That's why I don't understand the relevance of request about how to calculate the file size beyond curiosity. It's not that the mods are unsure about how to determine if a graphic meets the guidelines; that has nothing to do with the technical composition of what determines the size of a file.

Feel free to send me a link to a picture you are thinking about using if you're unclear about whether it meets the guidelines. I'd be happy to review it and let you know, or help you shrink it down to an appropriate size.