Evil Mad Scientist Laboratories sells the AxiDraw pen plotter in several configurations. AxiDraws are more expensive than other pen plotters, but have great software support and are fast and accurate.
The AxiDraw MiniKit 2 is their most compact plotter with a working area of 150 mm × 100 mm. This is slightly less in the short dimension of A6 paper, at 146 mm × 105 mm, precluding full-bleed artwork in A6.
The AxiDraw SE/A3 is their second-largest plotter, with a working area of 430 mm × 297 mm, just enough to cover A3 paper.