Not a clue. You can't even begin to guess a sizing based on users - they're a tiny fraction of your data and mostly irrelevant when it comes to data size.
Your data size is mostly determined by (number of issues * complexity * activity). To do even a basic estimate, you'll have to explain your setup and usage in quite a lot of detail.
What I would say is that the size of the database is utterly irrelevant today - the cheapest smallest hard disk you can find will have more than enough space for the database for a very long time (start with a couple of Gb allocated and then monitor the size - that will be far more useful because it will tell you how YOU chew up space, rather than some arbitrary and unrepresentative calculations)
I might worry about attachments though - they live outside the database, and your users can attach quite a lot if they try.
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