Putting an estimate on the file size can be difficult because this is really up to the settings that each user selects. The higher the resolution that you set the greater the file size and it is also depends on the number of devices you are outputting to.

The pages used for the flash version are normally vector and these range between 100-200k per page.
There is then the images that are generated for the iPad version.
If you wish to have offline versions packaged with the online version then this will also dramatically increase the size of the folder that will be uploaded.

Dont concern yourself with the folder size as it will not effect the performance of the magazine as we have a load controller inside the software that controls how much of the magazine is loaded at one time. This ensures that the users engagement with your magazine is maximised.