I'm tempted to say that by today's standards 100 GB is a fairly small database. At least it is not frightingly big.
Anyway, do you have a lot of free space in the database? Else you need to delete/archive old data. How you would do that, requires knowledge about the system, so you would need to ask in a Navision forum.
Then again, where I live, company are by law required to keep bookkeeping records for at least seven years. The law may be different in your country, but I think you check this with your business side before you start any data cleansing.