If you simply want a working OS, download the instead. It is still supported, faster, and more secure. Your Windows 8.1 key might still activate Windows 10 (though not guaranteed).
If you don't have a valid Windows 8.1 license, you can use the Media Creation Tool to create a bootable USB drive or download an ISO file: download windows 8.1 64 bit iso file
Click the download button. The file size is approximately depending on the edition (Standard, Pro, N, or KN). If you simply want a working OS, download the instead