Are there any historical accounts of the size of shields used by Byzantine cavalry around the time of Heraklios?
The Strategikon doesn't give a size, but says that the lancers when charging can cover themselves and their horses necks (IIRC) so are, presumably, a reasonable size.
When I first read that quote I thought it might describe an oval shield but don't think that has ever been overtly mentioned
Have a look at the shields shown on the Ashburnham Pentateuch in the illustration of Pharaoh's army drowning in the waters.
Roy