Spanish twin bed: 0.90 m
US twin bed: 1.00 m

Spanish "two twin beds": 1.80 m
US king-size bed: 2.00 m

A Spanish "habitación doble" usually means two twin beds. You will find them separetely in hostals and pushed together in hotels.

Spanish "cama de matrimonio" (before): 1.35 m
Spanish "cama de matrimonio" (now): 1.50 m
US queen bed: 1.55 m

Note that the "new" size of Spanish "cama de matrimonio" is quite common at private homes, but not at hostals, mainly for the lack of space. Therefore, if you book a room with "cama de matrimonio" you'll likely to get a 1.35 m wide bed.

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.