Well, I'm no art expert, but my guess is that she is closer to 6. Assume that the height of an end table hasn't changed that much in 4 centuries, which puts the one in the painting at 22 inches. She's twice as tall as the end table, making her around 44 inches - which puts her around 6 YO.
A 3 YO would be closer to the 3 foot mark, usually.
That's my guess but :dunno:
Edit: Well scratch all that. The painting is supposedly of Margaret Theresa of Spain, born in 1651, and painted in 1654 - so 3YO.
http://en.wikipedia.org/wiki/Margaret_Theresa_of_Spain