It depends on how much space the backup will take

This is the most accurate answer. Yes, the smaller capacity of DLT tapes would make them a poor choice for an unattended full backup. However, they may be fine for an incremental or creating a backup of a single file.

There are several considerations when choosing a tape for an unattended backup. The size of the backup to be performed needs to be taken into account. So do tape length and availability of media.