Not sure if the OP received their answer, but when making reservations it is best to ask for a double. If you get the confirmation for a "double" or "cama matrimonial" then you will be fine. However, the combination may vary by establishment. We were always surprised. Sometimes we ended up with 2 twin beds pushed together (not the best for romance), and other times it was something akin to a full size bed in the US - but seemed just a tad smaller. A couple of times we were pleasantly surprised and ended up with a king size! These were in the Paradores (except for Parador de Cazorla), which makes them another "go for it" in my book!
Just be certain to be clear that there will be 2 people and that you request a double room, and you should be okay.