The degree of "granularity" (David Allen's term) for your Next Actions is a balance you decide for yourself. I believe the way he put it was if you need more motivation/bigger picture view, put less detail, and if you need it to be easier to take the next step, put more detail.
For me, "mow the lawn" is a single Next Action, because it is lumped into one established process. Importantly, as you become better at things, your processes will encompass more. Mowing the lawn COULD become a project, easily, if I am out of gas for the mower. Even then, there are times when getting gas for the mower would be lumped into the process to carry out that Next Action.
The purpose of Next Actions is to make it clear what to do next--to take your next bite of the elephant. For me, painting a room could be a multi-step project involving trips to several stores, prep work, etc. To a professional painter, that has become a process.
It is more efficient to have your Next Actions encompass more because there is less writing, reviewing, checking, etc. However, it is more effective at getting us off our collective rear ends if the steps are smaller, because smaller tasks are easier.
Sometimes a project "Mow the Lawn" could have one very small next action: get gas for lawnmower, followed by a very large Next Action: Mow lawn. Sometimes I intentionally do this just to get me going.
I hope this helps. You can depend on yourself, by monitoring your RAM, to know whether you need bigger or smaller Next Actions.