Missiles:
LRMs are 120 missiles per ton of ammo (~8.33 kilos per missile)
MRMs are 240 missiles per ton of ammo (~4.2 kilos per missile)
SRMs are either 100 or 90 missiles per ton of ammo (10-11 kilos per missile)
Compared to real life:
Hydra rockets are 6.2 kilos each, ~1 meter long, and 7 cm wide (or ~1 yard long, and 3 inches wide) (~1.6 g/cm^3)
Folding Fin rockets are 8.4 kg each, ~1.2 meters long, and ~7 cm wide (or 40 inches long and 2.75 inches wide) (~1.8 g/cm^3)
Average density is 1.7 g/cm^3 for these rockets.
I am assuming the same is true for BT missiles
An LRM is therefore ~4873 cm^3, an MRM is ~2436 cm^3, and SRMs range from 5847 cm^3 to 6432 cm^3.
If you want exact values, you can use the cylinder volume:
V(Cyl) = length * pi() * (Diameter/2)^2
If you want approximates, use the rectangular volume:
V(brick) = L * W * H (where width and height are identical)
So assuming a LRM that is 8 cm wide, it would be roughly 76 cm long
That could be an interesting idea, where Missiles are actually square shaped (but with rounded corners to avoid extra stress) so they don't roll as far if they spill.