orbitalguy

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

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.