@meganL Spaniard here. I read about plus-size shops in Bilbao in a newspaper some time ago and I got few names, in case I ever go. I never went, and I don't even know if they're still open, but here they go, hope it helps?
Polita XL: https://www.facebook.com/Politaxl/
Orchid Curvy: https://www.facebook.com/orchidcurvy/
Sin límite: https://www.facebook.com/sinlimitemoda/
(FB sites because Spain is different and all that 
)