I am designing light control board using micro controller.As I want to make my circuit board smaller, So I am thinking to use Optoisolator or Optocoupler instead of using Relays. So please let me know, is it fair to replace "Relay" with "Optoisolator" or "Optocoupler"? Can Optoisolator work on more Amps?
If your load is AC AND low power then some opto-isolators will work such as the MOC3021. It has a triac output capable of driving a low power load in the order of a few mA but, more likely you would want to drive a more powerful device such as this: -
The external triac would typically be a BT136 and there are a pile of google images here that show this type of configuration. The BT136 comes in a T0-220 package that isn't that small but maybe some space saving can be achieved. A lower power BT131-600 could handle 100 watt loads but RMS current is limited to below 1 amp. It is available in a T0-92 package.
You could also consider using a solid-state-relay although a lot of them will contain pretty much the above Triac circuit or back-to-back MOSFETs: -