Resize line art image I want to print some line drawings on A3, but they aren't very large - around 600x800px.

I tried obvious functions in Gimp, like resize then enhancing with wavelet sharpener or unsharp mask, with selective gausian blur before it, but print results are no good.

They are not easy for vectorization either, but perhaps tweaking some tracing tool can give me better results then resizing bitmap. Before I start exploring this option, I thought to ask for advice:

Can anyone share a recipe, either by using programming algorithms (numpy/scipy or matlab) or by using tools available in Gimp or Photoshop, to accomplish quality resize of line drawing?
Take a look at pixel art scaling algorithms, particularly the hqx algorithms.  They're typically limited to a integer scaling factor (i.e. 2x, 3x, 4x) so they aren't completely general purpose but they should do quite well on line drawings.