I looked at your Mundaka sailing site on Goggle Earth, Looks pretty good.
How much current in in that river.... does it flow with the tides near the mouth of the
After checking out your sailing site, I am even more convinced that you need the bigger, higher volume board. I still suggest the Rio L or M or a Start L or M. You need the extra float at your weight.
Only way to tell if a smaller board will work for you, would be to try to rent a smaller board (maybe at the site where you took your lessons) and see how you do with something in the 150 liter range.
You might do OK, and then on the other hand it might be so "sinky" and unstable that you cannot sail it at all.
Only way to tell for sure is to try it, and if you buy one, and it's wrong, you are going to lose some of your investment trading it back on something larger.
Have you looked at any online shops there in Spain?
You might find something a long ways from where you live, or have to drive to pick up, or even have it shipped to you, but as popular as windsurfing is in Spain, there must be some large begnner intermediate boards available somewhere.
Hope this helps,