Where do you find the file size?
These can be found easily in any operating system. In Windows, the properties section in each file will show the size in the most appropriate unit for the life size.
In Mac OS X you can press Command+i to show details of a particular file, including file size. Most email programs also show the size of the attachments next to the file specifications. All FTP programs also show by default the sizes of the files transferred to websites.