Question: I am having a hard time evaluating how much size i need for my SQL2005 database. I am creating a content based website (articles - videos - images - forums ) with user profile and was thinking about having probably a small to a mid size audience.Is there a way to evaluate how much space or how many databases i would need ? I have an account hosted at DiscountASP and for the database size they offer (300 -700MB) i have the feeling that i would reach the limit pretty fast and will have to spend a lot more.

How do you calculate/test how much space you need? Does adding as many dummy entries to the database a good enough way to evaluate the database usage?

Answer: 
If you're only storing information about the videos and images in the database you shouldn't reach that kind of limit quickly.

To test this why don't you install SQL Server 2005 Express on your system and see how big your database is with an amount of sample data close to what you expect. Also, most hosting companies give you the option to move to a larger database later so if you think it'll be an issue make sure to go with a company that allows this.
Try to approximate. For instance, "comments" table with dynamic-width rows: avg-size = (uid [4] + flags [4] + avg-comment-length (100)) \* number-of-records = 144 bytes per post \* number-of-records = ...

The numbers heavily depend on your table structure.

Also, if you're designing a web application: think of exporting long data from the database into files to save space. E.g. comments can be stored in data/comments/.txt

Additionally, you can use compressed tables (or manually gzcompress text data defore insertion).

Cheers!