I am trying to do a clean install from Yosemite to High Sierra. The boot installer is only downloading 19.5MB instead of the 5GB's. Any help would be appreciated.

You need to create a bootable installer USB drive. Here is what you must do:

1. Download macOS Sierra from the Mac App Store

2. After the download has finished, plug in your USB drive (at least 8 GB or more). Name it MyVolume

3. Run the following command:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app

4. Type in your administrator-password. There are no digits showing that you are typing your password in. Press ⏎.

5. Press "y". Remember, all of the data on the USB drive will be removed.

6. Once the process has finished, shut down your Mac.

7. Boot your Mac up using ⌥ down.

8. Choose the USB drive called "Install macOS Sierra" as your startup disk.

9. Choose "Install a new copy of macOS". The installer will guide you through the installation process.

10. Have fun with your new macOS Sierra!