Prior to the mid-16th century, watercolor was primarily used for the painting of miniatures in illuminated books. These hand-painted and inscribed volumes were usually devotional, but sometimes were essentially calendars.
Perhaps the greatest and last Flemish master of this form was Simon Bening. He was a member of a family of artists. His father, Alexander Bening, was a painter, his eldest daughter became court painter to Edward VI of England, and another daughter was an art dealer.
The best examples, in terms of quantity and image quality, are on the Getty Museum site. There are 90 images. While some are illuminated pages of text with images around the edges, those at the very beginning and very end of the selections are full images. Once you click to the detail page for an individual image, look for the “Download” link under the image for the high-resolution version.
These paintings, done in watercolor on vellum, occasionally augmented with gold leaf, were tiny. Those shown above, at top, first two (each shown here with a detail) were on pages roughly 7 by 4 1/2 inches (18x11cm).
At the very end of the selections on the Getty, are two horizontal images that are roughly 2 by 4 inches (5x10cm), one of which is shown above, with detail — bottom two.
My favorite series, however, is the Labors of the Months, from a book of hours and calendar, accessible on Wikimedia Commons, though the images are not as high quality or high resolution. These are essentially a wonderful series of miniature landscapes, at a time when landscape was just coming into favor as an important subject. There is information about a facsimile of the book here. It is roughly 5 1/2 by 4 inches (14x10cm).
I love the rich, painterly quality Bening achieves with his watercolor (and/or gouache, I presume), even at the restrictive size in which he was working.