How much disk space does 'n' accounts or contacts take up?

A good point of reference is:

10MB of disk space can easily hold about 3500 accounts, with 2500 contacts, 1000 notes, and 1000 activities.

These are just estimates and can vary depending on the size of data stored in the fields and the size of notes etc...

Please remember that a database will be bigger than the text file you may have or Excel file due to space require for Indexes and other features that make databases so powerful.