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.
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