I want low energy consuming AC for my 10X20 Ft room. What should I do to cutdown room size so that small AC can also become effective?

A small window AC would do nicely.

Kul® 5,000 BTU Window Air Conditioner - BedBathandBeyond.com

Cut down room size? You mean build a wall? Sure, build a wall. You could even build one that is only tied down in 4 places on the side walls, not ceiling and floor, so that it is removable and patchable without too much work.
The key to energy COST saving is to buy an expensive, low amp AC. I cooled a whole first floor with an 8,000 BTU that is only 6.5 amps. But it cost more than double what those 12 amp ones cost. Worth it! And you won't have to make a smaller room.