Windows 10 ISO download size will depend upon what version you are downloading and when in future downloading,
Right now May 2019 Update of Windows 10 Size is as follows
Language English
win-10-iso-size.jpeg
You can Download Windows 10 ISO from Official Microsoft website from here
https://www.microsoft.com/en-in/software-download/windows10ISO
Keep in mind
Option to Download ISO will only appear if you visit this URL on mobile or
On Windows PC use chrome browsers developer option device toggle toolbar feature by the following shortcut Press F12 then Ctrl+Shift+M
device-toggle.jpeg