Has anyone caught the RER B at Cite Universitaire at 6:45 or 7:00 a.m. for CDG? I'm wondering if it will be crowded by then. If so I'll book airport transport.

Yes I have.
It will be moderatly crowded. Rush hour in Paris starts usually after 8 am (at least intra muros).

yes, I have as I lived there for a month and had to go to school at that time in central Paris--more towards the 7-7:30 mark. It's beginning of rush hour but that is near the end of the line (within Paris anyway), I wouldn't personally worry about it, I think you'll get a seat if that's what you are worried about. But if you don't care about taking something else, and have an alternate source you like, sure, book it.

I have not caught it at that location, but we regularly take the RERB to CDG and have left anywhere between 7 and 8:30 am. With carry on luggage we have not had any problems.

Thanks... we'll have carry-on but not alot and want a seats. I thought that was early enough to not be problematic.

No, it is well before rush hour which begins around 8 a.m. inside Paris.

And after passing Gare du Nord you are going out of Paris

PalenQ ... Cite Universitaire isn't the first stop on the route.. that's why I asked about space.

Still very early. Paris is a notoriously late starting city. And on the weekends there are hardly any cars on the street until past 10 a.m.