Spain...Help with Planning

If you have a car then, no, 5-6 days in Sevilla is not too much. Since you haven't been there before, a good two days will allow you to see Sevilla itself. Then a car will allow you to do enjoyable day trips to places like Arcos, Cadiz, and even Cordoba.,...all within 2 hours' driving time. The great Roman ruins of nearby Italica are also easily accessible by car, though you can catch a bus from Sevilla to do this journey. But to have a car while in Sevilla means staying outside the old section of town, where streets are VERY narrow, one-way, and very confusing....never mind the lack of parking.
I would certainly recommend a car for travelling Sevilla-Marbella, if only to allow yourself the freedom to explore some of the "white villages" along the way. I would suggest that, whatever you decide, you should try to spend an overnight in Ronda, and visit some of the surrounding villages like Zahara de la Sierra and Grazalema.
Even if you don't use a car for Sevilla-Marbella, there's no need to go all the way to Malaga. There are 4 or 5 buses a day Sevilla-Ronda, and the same number/day Ronda-Marbella.

What Norm sais is great. You might also throw something else into the mix if you think six days is too long in Seville and you're thinking of getting a car anyway.

Pick the car up in Madrid and do a swing through Segovia, Avila, and Toledo on the way down. It would be an easy two-day drive and most of the luggage could hide in your trunk for the one overnight stop.

We spent five days in Seville two summers ago and had a wonderful time. We took the bus to Italica -- which was a great experience -- but other than that, we stayed in the city and really took our time with things and got into the rhythm of Seville. There really is a lot to do and see and experience, like just wandering the narrow lanes around the Triana district. You can always plan for day trips as well, either by car or by public transport.

Thank you all for your responses. I am taking each very seriously. I wish we did not have to deal with the luggage issue but we do. I am concerned about leaving any luggage (even hidden) in the car overnight so I am trying to limit the stops. Glad there is much to do with daytrips.

Someone mentioned bus service as well. If we were to take a bus from Seville to Marbella or on any fraction of the trip will there be room for large suitcase? We took a bus in New Zealand and they did a tour and transport. It was great as we could load all of our luggage in the bus (the belly of the bus) and after the 6 hour tour we were in Cristchurch. Can this be done on these busses? Just checking all alternatives out.

Thanks again.
Karen

Oh, yes,...the intercity buses are large and comfortable (much better quality than the Greyhound service here) and can carry a ton of luggage underneath. You can check the bus schedules for Ronda-Marbella by clicking here. Journey time is about 1:15. I can't for the life of me find the Sevilla-Ronda bus schedule, but there are 5 or 6 daily, well-scattered through the day. Trip takes about 2 hours...any of the TIs will be able to fill you in. Double check just before your departure to ensure the latest schudule.
And a reminder that they have assigned seating.

I probably would allocate about (3) days to Seville. Believe the largest Gothic edifice in the world is there. It is also a great place for "side trips." We passed, due to time limitations on Cordoba but understand that it is indeed worthwhile, Granada and Malaga are also quite close. Granada was excellent; Malaga quite nice also. A little "gem" is Ronda, where the "art" of bullfighting originated. We normally travel by car but on this particular trip trained our way. Were rather "bold" and went down to Algeciras and took the ferry to Tangiers. Was interesting but I would not recommend it for independent travel; when we were there visitors were harassed by the locals. Being from Chicago, I hired a bodyguard and that was the end of that. Spain generally is wonderful; I would have no fear of driving.