Google photos handles RAW pictures at the moment (30/06/2015) like this:
If a jpeg preview is embedded in the RAW, the jpg preview is used to be shown in Google Photos, regardless of its resolution or whether you uploaded in "High quality" or "Original size".
If no jpeg preview is present in the DNG, the RAW data is used to generate a full resolution preview to be shown.
If you look in the folder "Google photos" in "Google Drive, only the preview jpg is available there for download, even if you uploaded in "Original size". The original RAW can however be downloaded from photos.google.com.
So these are options:
Embed a full size jpeg as preview pictures, this is possible using eg. Adobe DNG convertor and it is also a setting in eg. LR (Link)
Remove the jpg preview entirely. There are quite some people that aren't fond of this option as the preview is used by quite some photo management applications to speedup viewing,...
If you only want to upload in "High quality", you can consider first exporting to jpg as it will also save a lot of bandwith (look at the remarks below)
Remarks:
This has been tested with some DNG and ARW files, so I suppose it is the case for all RAW formats.
It has been tested using both the browser-based upload and the "Google Photos desktop uploader".
Whatever the circumstances (preview or not, "High quality" or "Original"), the RAW image is first uploaded completely, and then is resampled at the backend. So always the full bandwith is used.
More info about High Quality vs. Original and how it affects RAW files can be seen here:
Google Photos "High quality" vs "Original": What's the difference and should you care
Shortended link to this page: https://goo.gl/i3dJaO