Let do some simple math. I'll use my Touch.
Horizontal resolution: 600 px
Vertical resolution: 800 px
Diagonal: 6" screen (15.24cm)
Display size: 3.6" × 4.8" (9.14cm × 12.19cm)
Source:
DPI Calculator / PPI Calculator
My own height measurements of header and footer (using a measuring tape)
Header: 0.80cm
Footer: 1.20cm
Total height of header and footer: 2cm.
The header and footer use (block) 16.41% of the screen. Just 83.59% is available for reading.
The actual number of pixels available for reading:
Height: 669 px (83.59% of 800px; rounded)
Width: 600 px (100%; assuming that the left and right margins are zero. Which is not the case when reading kepubs)
Diagonal: 5.39 inches (13.69cm)
Display size: 4.01" × 3.6" (10.19cm × 9.14cm)