If you want to buy a tablet PC, the screen size is critical, it is related to the comfort of your eyes. The most obvious difference between these different tablet PC area is the size of the entire screen, and there are many varieties. The screen size is the distance between the screen diagonal.

For tablet pc, there has 4 class to divide the screen size:

Over 10.1 inches tablet: is belong the larger screen tablet pc.
this Screen tablet is to replace the laptop for watching videos, HD movies, play games. such as:
Toshiba AT330/003 13.3inch
Dell XPS 12 XPS 12 12.5inch
Asus EEE Slate ep121 12.1inch
Fusion Garage JooJoo 12.1inch
Lenovo IdeaTab Lynx K3011 11.6inch

The largest screen of tablet pc is Acer ICONIA-6120, which has a screen size of 14 inch.

the 8.9 to 10.1 inches is belong the medium screen tablet pc
Larger screens are better suited to movie watching, and certain games will benefit from the increased real estate as well.

such as
Apple's iPad 9.7inch
Samsung Galaxy Tab 10.1 10.1inch
ASUS Eee Pad Transformer 10.1inch
Motorola Xoom 10.1inch

The 7 to 7.9-inches is bleong the small screen tablet pc
small tablets are much more portable and usually fit more easily into purses and small bags. Some can even fit into pockets, depending on their width.
some china tablet pc with 7 to 7.9-inches screen

Newman T7 / Newpad Black G27 7inch
S7-301U 7inch
Onda VI10 / ONDA Vi20W 7inch

The 5.0 to7.0 inch is bleong the smallest screen tablet pc:

Another whole class of tablets have smaller 7in screens such as the original Samsung Galaxy Tab, BlackBerry PlayBook, Dell Streak 7 and HTC Flyer.

But there are some tablets that sit outside these dimensions. LG's Optimus Pad, for example, has an 8.9in screen, and Samsung has a range of different-sized tablets that include 7.7in and 8.9in offerings as well as the 5in Galaxy Note. Meanwhile, Sony has the Tablet P, which features two 5.5in screens that fold up to make a clamshell device.

Another screen Knowledge is Display resolution.

Screen resolutions vary according to the manufacturer and the price range in which the tablet is sold. From the QXGA resolution on the New IPad to the WVGA resolution that is found in the most basic models, there is a wide choice available for the consumer depending on his or her personal needs.

Actual size is only part of the screen factor. The iPad 2 with its 1024 x 768 pixels is different to the Samsung Galaxy Tab 10.1 and Asus Eee Pad Transformer with their 1280 x 800 pixels. The BlackBerry PlayBook and Samsung Galaxy Tab cram 1024 x 600 pixels into their 7in of screen area. If all this sounds complicated, rest easy. All the screen sizes and pixel resolutions quoted are perfectly good for everyday use. High-end tablets place a significant emphasis on good screens and it's only when you look at the budget end of the market that you might begin to see lower pixel resolutions, which could make for less satisfactory use, particularly for text or graphics intensive activities such as video viewing.

Typically, the screen resolution can be categorized as follows:
QXGA: 2048 x 1536
WXGA: 1366 x 768 or 1280 x 800
XGA: 1024 x 768
WSVGA: 1024 x 600
WVGA: 800 x 600
Of course, there are other screen parameters:
Screen coating, Brightness and Protection, Screen Viewing Angles, Screen Technology,etc