The data volume corresponds approximately to the size of the media files (e.g. images, videos, audio) you integrate into your intervention. Based on our experience, I would recommend to integrate audio and video clips from professional portals, because they deliver the content in the optimal format for the respective mobile devices (e.g. depending on the bandwidth of the connection). So I you may use the
show-web URL command in MobileCoach Designer and link to, for example, a Vimeo URL.
See here for a video tutorial: https://vimeo.com/404368142
Finally, I would add about 20% overhead of bytes (for the text messages, etc.). But, as always with technology and mobile applications and streaming of media content, I would also test it with real devices before you deploy your intervention to your participants.
I hope this helps.