In my experience, the BH database is pretty compact. If you have a lot of audio or PDF attachments, those will take up the majority of space. In my case I have something in the neighborhood of 300 songs with a total of 700ish attachments. The total space taken up is 830 MB.
The attachments are copied to the device as separate files, in their original format, without compression. And the entire database is downloaded to the device.
If all you will be using the tablet for is BandHelper, I'd say you would be good with the 64MB model.