To change the scale of a character, select the 3D model in the Project window in Unity so you can see its import settings in the Inspector window.
Under the Model tab, find the Scale Factor field and adjust the value to your desired scale for the character in meters.
When you have the scale you want for your character, click Apply to apply the scale. Your character should now be the correct scale when you set it up for importing into Flipside Studio.
Note: Don't change the scale of a character in the Hierarchy window. Instead, follow the steps above to set its scale. Otherwise, this will cause problems with some features in Flipside Studio.