Oblivion game units
From Nexus Wiki
When creating models, you might want to size your model to specific dimensions...which requires knowing how the length in the game translates to measurements in your modeling program.
Length of Oblivion Game Units (GU)
1 foot = 21.3 GU 1 inch = 1.775 GU 1 meter = 70 GU 10 centimeters = 7 GU
Blender Units (BU)
There is a 10 to 1 ratio between GU and BU.
1 foot = 213 BU 1 inch = 17.75 BU 1 meter = 700 BU 10 centimeters = 70 BU
3D Studio Max Units (SU)
There is a 1 to 1 ratio between GU and SU.
1 foot = 21.3 SU 1 inch = 1.775 SU 1 meter = 70 SU 10 centimeters = 7 SU
Maya Units (MU)
Unknown. But since it is made by the same people as 3D Studio Max, it might be a 1 to 1 ratio as well.
You have a weapon whose length needs to be 40 inches in-game. The model should measure to be 710 Blender Units or 71 3D Studio Max Units.