As I said, 16x16. 8x8 is ok but cutting it close, and again, the differences are negligible.
Honestly in most cases, if I’m making a simple background asset like this where the player isn’t handling or interacting with the object and I just need a flat diffuse texture with no detail, I do as Eurocracy suggested above and export the foliage and trunk of the trees separately, and colour them with BrickColor.
The performance hit of two meshes verses a single mesh and 16x16 texture is also negligible and at that point you’re better spending time with other performance methods.
I always try to allocate the most resources (time, money, game memory limits) to the more important assets. Why spend time to making a texture of four colours when I could just export the minor asset and colour it in Studio?