This is an example of the same material on the ground and on the rock above it. When applied to the rock, it changes the size of the textures. How do I adjust this?
You need to use a TextureCoordinate node to change the tiling size of the texture uvs.
I’ve never done that. Any info to get me started?
Hi Sladus,
Take a look at this documentation on different nodes in the Material Editor.
The texture coordinate node information is near the bottom of the page.
Essentially what this node does is plug into the UV section of your texture sample and you can set a tiling value for U and V. To evenly tile the grass material you will need to evenly adjust both these values.
If you have any further questions about using it please feel free to ask! 
Tim
I checked and it has several texture coordinates. It is M_Ground_Grass. It comes with UE4. Do I need to make changes to it?
Yes, you need to increase the tile values of those texture coordinates.