If you want to change settings in the launcher, change icon size, or grid size just hold down on an empty part of your home screen. Tap the three Dots and there you go.
From https://forums.oneplus.net/threads/how-to-change-icon-and-grid-size-trebuchet-settings.84820/
When configuring the phone for first time I saw something about a grid somewhere, but couldn't find it again. Luckily I found the answer on the link above.