With your Notebooking Pages LIFETIME Membership, remember you never lose access to these files, so there is no need to download everything to your computer. However, this is a common questions, so here's the answer:
The LIFETIME Membership would require at least 1 GB of space on your computer.
I would recommend downloading/transferring the files to a USB flash drive (5 GB ~ $10) which would make the files easy to share between multiple computers in your home. If you would like to transfer the files to multiple flash drives (perhaps one for each of your children), you have my permission to do that as well. Our copyright allows you to use the downloaded pages/files with your own family and a single classroom that you teach (EX: a co-op class).