PC Games Hardware has compared the power consumption of 30 LCD displays; the test features 22", 23", 24", 26" and 30" LCD displays. For comparison, the tester also throwed in two 19" CRT displays. You can check out the results over here.
Generally, the larger the size of your display the more power it will use, but the test shows that some displays use more than twice as much power as one of the same size. The 30" Dell 3008WFP uses most energy, this screen consumes 140W while the 20" ASUS VH226H uses only 20W. There are also big differences in standby power consumption, several 24" models use 1W while some smaller models use as much as 10W in standby.
The results also show CRT displays use significantly more power than LCD displays, the 19" CRTs that were tested use more power than most 24" and 26" LCD displays, but not as much as the 30" LCD displays.