The average screen size are (in pixels):
480x320 (iPhone 1st, 2nd and 3rd generation)
800x480 (generic Android, Samsung often much higher)
1024x768 (iPad 1st generation)
iPhone5, iPad2/3 and most Android tablets have resolution close or better than many traditional computers.
Quick comparison: Toshiba Tecra M1, built a decade ago: 1024x768. iPhone5: 1136x640.
Mobile devices use the HandHeldFriendly tag as an indicator that the page is mobile-friendly and the web document should be displayed without scaling.
<meta name="HandheldFriendly" content="true" />
That tag is not suitable for landing pages intended for mobile and computers.
The Viewport meta tag is a safer approach:
<meta name="viewport" content="width=device-width; initial-scale=1.0; maximum-scale=1.0; user-scalable=0;" />