After I got it out of the house, I measured the distance from the picture frame to the mark on the canvas visible in the image, thinking it might help with scale - approximately 35mm.
Off the top of my head, given the size it's a pipistrelle and given the dark face probably a Common Pipistrelle (P. pipistrellus), though you'd need to take measurements or check the echolocation frequency (or take a DNA sample!) to be sure.