The size strongly depends on the number of people analyzed since the Toolkit dumps raw data without aggregation. S single file for the Toolkit processing a video of a full crowd for 30 seconds generates around 30kb of data (excluding landmarks). Interpolating the above to a full day of data, this would mean a CSV can generate around 60kb of data per minute, which is around 3.6MB per hour and around 86MB for 24 hours. That would be around 30GB of raw data in a full year.
The Toolkit does not do rotation on these files (removing old data and write new data to never exceed for example 1GB) by default so once you start writing to CSV, you need to make sure you have enough space on the disk depending on how long you want to run the software unattended.