Creat an object to the RIGHT scale you want then export it out to the modelling app your using and import it into that to get the correct scale. Then scale your original model to the size of the reference object and export to the results you want.
The import into Med should then be as it should.
Perhaps see what you are using in your modelling app for distance measures i.e. feet, yards, metres, cm, and so on and compare it to the relative scales in MED.