Roman mosaics were usually made of small square tesserae of 10mm x 10mm (3/8" x 3/8") size.
Below are a number of links/images of extant Roman mosaics, most of them quite elaborate. It seems doubtful that our Libertus did much more than geometric patterns –which could be very intricate though – but may have had other design templates. Occasionally, the mosaicist would sign his work, for example, The artist at work.
In my travels in 1999 through Germany, I went to Bad Kreuznach near Mainz, where two mosaic floors were discovered in the remains of a Roman villa, in 1893 and 1966, and are now preserved in a museum barn. They are presumed to be from the 2nd century AD, the period in which the Libertus mysteries are set. So they are quite appropriate for our upcoming chat on February 7.
The floors were heated by a hypocaust.
Bill Thayer has a page on the Domus from Smith's Dictionary of Greek and Roman Antiquities, John Murray, London, 1875 . (excellent!)
Nova: Lost Roman Treasure (including another Oceanus) from Zeugma, now under a new reservoir on the Euphrates River
Roman Mosaics in Tunisia
Also from Tunesia:
brooklyn museum: jewish mosaics
from an exhibit in Cologne
The history of mosaic art - Mosaics in the ancient world
Wikipedia: Mosaic, Tessellation
Make a Roman mosaic online from Roman mosaics at Roman Britain / Examples
The Association for the Study and Preservation of Roman Mosaics (ASPROM)
Images © Irene Hahn