Let's talk about how to control film grain size in Exposure. First, I'll acknowledge that individual film grains can only be seen with a microscope. The little fractal lumps you can see in film are clusters of many grains. Still, let's use the term grains to refer to those visible lumps, since that is what most people mean.
The bottom of the Grain panel is where you control grain size.
The Automatic checkbox is on for all of the factory presets. In this mode, we use a model of your image as a piece of physical film. If you scan a piece of film, the number of grains is the same regardless of the scanner resolution. Similarly, in automatic mode Exposure chooses the number of grains based only on your choice of film stock and format, not the number of pixels in your image. The grain pixel size is adjusted to achieve the proper number of grains.
Suppose you apply an Exposure factory preset to 6MP and 24MP images and then print them both at 8x10 inches. They will both look like they have the same grain. That is the purpose of automatic grain size. If you use Photoshop to zoom to 100% then you will see that the grains are twice as many pixels big in the 24MP image, but when printed the grains are the same physical size.
The factory presets use 135 format (36x24 mm), but you can choose medium or large format film to decrease grain size.
You can also adjust grain size with the Relative Size slider. As a bit of trivia, 1.0 happens to be Fuji Reala in 135 format, but that choice was arbitrary.
If you turn Automatic off then you can directly control grain size in terms of pixels. It is easy to understand the concept that your grains are 2 pixels big. However, in this approach your grain size does not increase when you go from working on a 6 Megapixel image to a 24 Megapixel image. The 24MP image will have twice as many grains in each dimension. The result is that both images printed at the same physical size (8x10 inches, for instance) results in the 24 Megapixel print having much smaller (and less visible) grain. That is why Automatic is on in all the factory presets.
There is one caveat to the automatic grain size system. Grain size will never get below 1.5 pixels. Below that size, grain becomes digital noise with no clumpy natural quality and that's just ugly. This means that images below about 3 Megapixels will usually be stuck at 1.5 pixel grain.
In real film, if you make a huge enlargement then you will not see any detail smaller than the grain size. With Exposure that is not true. If you zoom in on a very high resolution photo then your original image detail is there under the grain lumps. It would be more realistic to give the image a blur at a radius about half the size of the grain, but we chose to leave blur out of most presets. Such destructive operations are best left to the photographer. You can do a very small blur in Exposure's Focus panel. However, I wouldn't do a blur unless you have turned on very strong grain, are printing at a very large size, and really want to create the blurry feel of a film enlargement.
If you frequently adjust grain size, then I encourage you to save some presets. That is an easy way to get consistent results.