Like adult diapers. It depends.
If you are comfortable with how things look when your project is stretched larger or smooshed smaller than it was created, just enable the Scalable option in Captivate.
If you want it to be pixel perfect, you need to survey your audience or ask for a report from your web peeps about the most common resolution folks are using. Then design the Captivate to fit within the topic area for that resolution.
Unfortunately, this makes me feel like a politician evading a real answer and I must now go shower!
The fact is, there isn't a pat answer here. It's quite literally like asking how long a piece of string is.
Thanks Rick, I guess it's just a matter of trial and error.