1) Is it possible to read passive tags at a distance of 6m
That depends on the field strength required to activate the passive tag. That should be a part of the passive tag specifications, and is your starting point for the reader design. Generally, the answer is , yes.
2) Is it possible to build this reader.
Yes. However, you will need the passive tag specs before you can start the design.
3)What would be the size of reader
I suspect it would be easy to put in a cigar box unless you need a specific beamwidth for the antenna. It depends upon your application, and what type of signal processing is needed in the reader, i.e. does it merely pass the data to and from another external computer.
4)What would be the power requirement of this reader
It must supply activation energy for the passive tag at the required distance. That's why you need the tag specifications. You also need the frequency, since it will be different for all of the possibles ( 140khz, 13 Mhz, 915 Mhz, 5800 Mhz)
5)Is it possible to detect direction and distance of RFID tag from the reader
( It depends on the spec requirements. Distance is very difficult and I would say probably not. Direction can be done using directional antennae, which rules it out pretty well below 915 Mhz.) A 915 antenna such as the TIRIS antenna is about 5 feet long and 1.5 ft wide and will support beamwidths of 25-30 degrees..... approx. ball park.. 5800 directional antenna are about 6 inches in diameter if made with multiple patches and microstrip................... Don't even think about using short pulses for distance --- passive tags can't do that by a long shot and a reader design would be impractical )
Andy in Eureka , Tex ( retired RFID engineer )