According to The Met Museum website, the mosaic Emperor Justinian and Members of His Court measures 104 in. by 144 in. or 264.2cm by 365.8cm, which is 8ft 8in by 12ft. The same dimensions are given for the mosaic Empress Theodora and Members of Her Court.
Slightly less precise measurements of 8ft 8in by 12ft, or 2m 64cm by 3m 65cm are given in this source for both mosaics.
A third source, Imperial Gifts, has both mosaics measure slightly smaller at 8ft 6in by 12ft, or 2m 59cm by 3m 65cm.
As can be seen in the image below, the mosaics face each other in the church's apse. They date to approximately 547 AD. More details can be found on The Met's Dress Styles in the Mosaics of San Vitale which also notes that:
In both portraits, the rulers are dressed to identify themselves with
the saints and prophets surrounding them in mosaic decoration, and to
establish their eternal presence among the divine.
The apse of San Vitale showing the two mosaics on either side of the three windows, with Justinian on the left and Theodora on the right. As suggested by Lucian in a comment above, the people portrayed appear to life-size. Image source: akg images