Using a 400 dpi scan of the letter, I could determine that its overall size is:
W = 21.1 cm , H = 30.7 cm
Of course, DIN A4 had not yet been invented, but this turns out to be very close to that size.
Now, using the next image:
- the width cf. the red arrow is 15.3 cm
- the height cf. the blue arrow is 22.2 cm
- the height cf. the green arrow is 22.9 cm
The size of the MS is W = 16.0 cm , H = 22.5 cm
This means that the letter, when folded along the lines ending at the red arrow (vertical fold) and the blue arrow (horizontal fold), would fit exactly in the MS.
While I am not clear how the wax seals figure into this, I have no doubt that this letter was once glued to the inside cover.
With respect to the modern cover, that was added (replacing an older cover) by the Roman Jesuits, traces of paper remnants on it clearly suggest that whatever was torn away by Wilfrid Voynich was not the Marci letter.
The other fold lines may be from refoldings by the Jesuits, or even Voynich himself.