[Smallest rocket capable to send human being into orbit???]

Mercury-Atlas specs

Size

Height 28.7 metres (94.3 ft)

Diameter 3.0 metres (10.0 ft)

width over boost fairing 4.9 metres (16 ft)

Mass 120,000 kilograms (260,000 lb)

Stages 1½

-from Wikipedia http://en.wikipedia.org/wiki/Atlas_LV-3B

[Which spacecraft was better Mercury or Vostok]

I would have to say Vostok. It could last longer than mercury and was modified to support up to 3 crew, that was the Voskhod. Mercury, however, barely lasted a day and a half, and it was safer. The only real reason the USSR could do it was because the nukes they had where so heavy they needed extremely powerful ICBMs.