Short answer: ad durations are fixed by format; source videos can be longer and get clipped.
Ad durations
Video variants render at standard spot lengths — for example CTV and OLV at 15s or 30s, selectable per variant in Ad Builder.
Longer source videos
Source videos longer than the ad duration are clipped to fit. Two ways to control it:
Pick the segment yourself on the Setup step, or
Use Smart clip and we pull the strongest segment automatically.
[CSM: confirm max source upload length/size before publish]
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