A question from a student made an interesting point. She was working on a marble version of a border section of a smalti mosaic of a large bush/tree growing up from a basket (a fairly well known one in Ravenna). There were apples on the tree and she wanted to know if it would break the Rules if she used smaller tesserae to show more detail in the apples, this was my reply:
'In reply to your question about using smaller tesserae for the fruit the answer is yes, you would break the Rules. In a number of the North African mosaics you'll see the figures set using smaller tesserae, they use these for the whole figure and not just the face either.
If you did that with just the fruit then they might become a point of focus, I.e. They draw in the eye, plus you're making more work. This, though is a part of a border pattern and so everything in it should be of the same size. What you learn by using all the same size tesserae is to decide what are the important parts of the image, say the red, the lighter areas showing light reflection, and then you learn how to replicate these areas using a lot less tesserae.
This is an important skill which allows you to reproduce mosaics that have been done with more tesserae than you have space for. It's learning what is the minimum size you can take mosaics down to'.
The thing to note in this is not just whether you can do it or not but also the skill you develop by working with what you have and not altering things to, essentially make it easier for yourself.
Here's the image of the mosaic from the mausoleum of Galla Placidia in Ravenna.
The same sizing of tesserae can be easily seen when you compere the apples and the leaves.
There are some smaller tesserae used in the rim of the basket but this is a later period mosaic in which they obviously felt they couldn't create the effect they wanted without doing that. These tesserae are longer, more rectangular in shape.