In a 3d editor you can see the size on a grid, before you convert it to a .ddd file.
The file conversion should make a perfect to-scale rendering.
You might need to turn on the grid feature.
Which program did you use?
On the otherhand, if you create it in glbasic language, you should be able to just whip up an algorithm to measure your object by subtracting the most extreme ranges of vertices.