From this web discussion (very last post, at the time I read it), a poster named gcook17 shares:
Sherry Yard's book, "The Secrets of Baking" has a table on page 233
with how many brioche molds of different sizes you need for 2, 2.5,
and 3 lbs. of dough. She gives the mold size in inches which I take
to be the diameter of the top. That assumption has worked out for me
Here are the number of molds for 2 lbs. of lean brioche dough that she
gives for several sizes:
3" - 21, 3.5 - 16, 4" - 11, 5.5" - 8, 6.5" - 4, 7" - 3, 8" - 2
When I use these they work fine, but I find I could probably put more
dough in each mold (i.e. use fewer molds) than she recommends.
It sounds like you have essentially an 8" brioche mold (and they are not exact measurements anyway), so 2 lbs or approximately 0.9 kg of dough is indicated to fill two such molds, so about 1 lb or 0.45 kg of dough for a single mold.