If you want to take the most direct route from Bordeaux to San Sebastian then there is only 1 route you can take. You'll take the A63 out of Bordeaux, which will turn into the N10 and then will change back to the A63.
Use the website viamichelin.com to get time and distance estimates as well as toll and fuel costs. The times given don't consider bad weather, road construction or traffic nor do they consider stops (fuel, food, bathrooms). The viamichelin site shows the above outlined route as about 2-1/2 hours travel time.
I don't know what your request for "no steep terrain" means nor do I understand your request for descriptions of the roads. If you want to know what the roads look like you might try google street view (provided they have this route covered). That being said the terrain is extremely flat coming out of Bordeaux to around Bayonne where there will be some rolling terrain but I doubt (if memory serves me well) you'll encounter any steep terrain on this route.
Edited: 26 August 2011, 0:44