This article is from the Chihuahua Breed FAQ, by email@example.com (Melinda 'Bob' Casino) with numerous contributions by others.
Chihuahuas are quite the sun-worshipers. They prefer to bask in the
sun for hours and have been known to lie in a spot of sun no larger
than the size of a half-dollar (1).
Unfortunately, they are not sensible in this regard and will stay in
the sun even when panting begins. Watch your Chihuahua in hot weather
to be sure that they don't suffer from heat stroke.
(1) Pisano, Beverly (Ed.), Chihuahuas (p. 9), 1983. (see Resources).