You could use the tape measure to find out how long the model is, then if you know what length its meant to be you can work out the scale factor you need to get it back to the correct size.

For example, say its 1.25m in the red direction and its meant to be 1m, then if you scaled by 0.8 in the red direction itd be the correct size. Rinse and repeat for the green direction.

You can work out the scale factor using length it should be/actual length.