Carat
 
Carat
is a weight
not a size measurement

Carat means how heavy a diamond, is not how big or large.

1 carat is 1/5th of a gram.

If qualities are equal - the heavier a diamond, the more it costs.

one carat = 100 points = 1.00ct
half carat = 50 points = 0.50ct
quarter carat = 25 points = 0.25ct

if cost is an issue:
to get the biggest diamond, sacrifice a bit on colour, differences are hardly noticeable and go for an I1 to SI2 clarity.