A presentation in presono is not a file and therefore does not actually consume any disk space. As soon as you work online in your browser, the data is all on our virtual server, which means it does not take up any space on your computer.
If you use the desktop app, however, you need disk space. The desktop app, as well as the mobile apps, offer you an offline mode and for this you have to download the data. This happens automatically and afterwards you can use your data offline. What needs the most storage space are the downloaded media files. But don't worry - in presono there is a maximum upload size, which ensures that this amount of data doesn't get too big.
Find more here: Present offline