That's too bad
Dorado's way is the perfect solution!
But, if you do the following math, it will give you an approx.
Length x width x depth = volume in cubic inches
so :
2 1/2 x 3 1/2 x 1 1/8
*) = 2.5 x 3.5 x 1.125 = 9.85 in3
*): it is best to take the smallest depth, because the forms are not square.
1 Gallon [Fluid, US] = 231 Cubic Inches
The party is yours from here on, because I've grown up with the metric system
Well ok, Ill do my best:
In cm and millilitres it would be
6,4 x 8.9 x 2.9 cm = 165,184 cm3 = 165,184 ml = 5.58 oz liquid US
all those conversions make it less precise; it will be between 5.4 and 5.6 fluid oz.