The Media Creation Tool will create the ISO for you, you need to check the option marked "Create Installation for another PC' and then check the box marked create ISO or Create Flash Drive, whichever you need . . .
The download sizes will be as answered above - if data is an issue, make sure you download the correct version (32bit or 64bit), don't select both if you are worried about download size . . .
Sanwin2 is correct, the new Media Creation Tool contains the Creators Update Version (1703), and that would be the best version to download . . . if you are using an older version of the Media Creation tool,
Click HERE and click the 'Download Tool Now' button