The new Google Photos service supplies "Unlimited" space for photos 16 megapixels and under. However, if I choose to upload photos at "Original Size" that are already at or under 16 megapixels, will they fall into the "Unlimited" space category? Or will they consume my limited quota?
It's not just about megapixels.
If you upload photos with the "Original Size" option then they will consume your limited quota, regardless of the megapixel dimensions of the photo uploaded.
This is because, under the "Original Size" option the original compression ratio is also used... the photo stored is the actual photo you uploaded.
Under the "High quality (free unlimited storage)" option all photos (including < 16mp) are resampled at a higher compression to save file space. This can affect the quality of the image (although most people probably won't notice). When photos come off the camera they generally have very little compression applied. The file size can be greatly reduced by resampling at a (slightly) higher compression with very little degradation in quality. However, a reduction in quality could be dependent on the type of photo used.
From the Google Help docs:
Choosing a storage size
What if I choose "High quality"...?
If the camera takes photos with 16 megapixel (MP) resolution or lower, most stored photos will essentially look the same using either storage option.
Note they are not the same.
Can I change my mind?
Changing the setting will affect storage sizes in the future and won't resize items that you've already stored.
"resize" also implies "resample" in this context.