First you need to find out what size the mosaic would be if you completed it using the same number of tesserae as the original, effectively making a piece for piece copy.
The main thing to remember that in Roman mosaic work your unit of measurement is a tesserae, not inches or centimetres.
1. Find an area on the mosaic that you have a clear, straight line of either 5 or 10 tesserae.
2. Measure this line and then get your width and height of the full piece. Here it is 65 tesserae by 45 tesserae.
3. Work out the average size of the tesserae you use (I'm working in millimetres here but it's the same for inches). My tesserae are slightly irregular, each side is 8mm - 12mm so I use an average measurement of 11mm. You
must remember that this will include something for the gaps between tesserae.
4. 11mm x 45 tesserae = 495mm, 11mm x 65 tesserae = 715mm. So if I copied this mosaic using my size of tesserae and including the same number of lines then it would be 715mm x 495mm.
This gives you the correct estimated size, don't try to be exact. Now we move on to how to reduce it in size.