Note, this is for SMP v7.5.
While shooting your animation:
As a guide, go to the Settings > Advanced Settings menu item.
Select the button : Overlay
Choose from the TV Save zone panel, a 4:3 aspect ratio option, or 16:9 if you wish.
go to the View Menu
select Mask to Safe Zone aspect ratio
This will blank out the non 4:3 or 16:9 areas.
Now, when you export your movie, File > Make Movie
go to the Size panel From the top drop down menu, choose:
Use TV Safe Aspect Ratio
and, from the bottom one:Crop to size.