Thanks go to Bill Chan for this one.
WARNING: Incorrectly editing the registry can render your Windows unbootable.
There is a registry setting available for setting the 3D SmartDimension Font type and Font Size:
In DimensionFont you will find a FontSize key, right click it and chose Modify, Decimal then enter your desired font size.
To change the font type you will need to add a new registry key to DimensionFont.
Below FontSize, right click in the blank area, New, String Value. Enter "FontName" as the name of the key. Then right click and Modify the key, enter the name of the font you want to use.
Restart IronCAD and it will use the new settings, enjoy.