Sensor Sizes

Although there are a number of sizes of 'chip' used in digital cameras, they fall into four areas,

Compact cameras use the smallest chips, ranging in size from 1/1.36" the smallest to 2/3" the largest of the 'compact' chips.
4/3 - Four Thirds
SLR cameras use either 'APS' size or Full Frame - same size as 35mm film.

In general the larger the chip the more pixels it can accommodate without the image becoming 'noisy'.

 

On the left is a crop from a compact camera chip and on the right from a full frame chip both at 400 ISO 100% magnification.

Generally more pixels mean bigger image, bigger pixels mean higher quality image. To have both you need a larger sensor.

       

Type    Sizes in mm
1/3.6"    4.00 X 3.00
1/3.2"    4.50 X 3.40
1/3"    4.80 X 3.60
1/2.7"    5.40 X 4.00
1/2.5"    5.76 X 4.29
1/2.3"    6.16 X 4.62
1/2"    6.40 X 4.80
1/1.8"    7.18 X 5.32
1/1.7"    7.60 X 5.70
2/3"    8.80 X 6.60
       
Type    Sizes in mm
4:3    18.00 X 13.50
APS-C   22.70 x 15.10
Full frame   36.00 x 24.00