This is how I installed the Pixel Launcher on my OnePlus One running LineageOS 14.
First, download the Open GApps Stock variant on your phone.
Second, create the following directory and file on your phone:
Put the following in it:
This will basically tell GApps to install the Mini variant and the Pixel Launcher, since there isn't enough room to install everything included in the Stock variant.
Reboot your phone to recovery, and install the GApps Stock zip file as usual.
More info on configuring GApps is available here.