What is HVGA Wallpaper size?

HVGA mean Half-size VGA which is screen that have 3:2 aspect ratio, in other words screens with a size 480x320 pixels. The former is used by a variety of PDA devices, starting with the Sony Clié in 2002, and standalone PDAs by Palm.
HVGA is a supported resolution at the first stage of Android OS up to version 1.5 when it's been introduce into the industry.
The most popular device that have this HVGA screen size is iPhone. The latter is used by HTC Lagend and HTC HD Mini.