Here are 2 simple charts on Carat Weight and Size.
Carat weight and size are important when it comes to assessing or appraising the value of diamonds. These are not the only qualities used to asses the value of a diamond but they do count for a lot.
Here is a graphical representation of diamond weight. In all cases when checking a diamond for weight, it should be weighed properly with the correct equipment.
Some more information on diamond weight:
Here is a pictorial representation of the size of a diamond from half a carat up to 4 carats.
(Of course there is no guarantee these are exact due to browser and image size considerations. One should always have a diamond properly assessed or appraised by a qualified diamond appraiser)