How can I use Mesh as a Tile on Geometry? I am currently developing a scene and some assets to work with. 

I'm not using any UV mapping or texturing as I'm going for an old polygonal 90's look. 

I have a 'Grass' mesh that I want to use as a tile on the faces of a simple plane geometry and I was wondering if there is a way to have the 'Grass' mesh scale to the faces appropriately. 

I was thinking of just simply Shift-D to duplicate the grass and then place it on the mesh manually, but I would like to know if there is a better way. 

Would be nice if there was a way to just extrude or scale the 'Grass' mesh and have it simply divide its appearance as it scales.
See Duplication and Particles/Hair.  I am trying to give a quick answer to get you started on research.  

Some of these have scale to face area features.

See this 
Tapering a tentacle mesh with uniformly scaled suction discs

Particles can have texture in (Strand/Particles) coordinates. The texture can control size of particles.  The texture may be a superior and more forthwith solution rather than size of tiles.

This proposed answer is not a tutorial. Seeing duplication and particles may be fun. Please search at BSE and the famous video site that rhymes with noonoob.   Keywords Blender Particles Duplication Dupliverts Duplifaces.

These concepts can involve some hours of work as you become more skilled with them.