We have 5 adults and 1 child in our group. I'm still debating whether to rent two small cars or one large car. I'm worried that a 7-seater might be difficult to drive in some places ?

Of course a car

We six adults...are ready to rent a 7-seater.

If you are confident with your skills, a minibus should be fine. If not, I suggest two small cars. I went and observed, and in Spain, there are more small cars. Only some engineering or company vehicles are small business cars.