My most elaborate - and time-consuming - project to date: the triptych cover artwork for the three volumes that make up the third book in The Night's Dawn Trilogy, by Peter F. Hamilton, The Naked God (Zeul adormit).
This bad boy clocks in at just over 11.000 pixels per 3912 pixels, which translates to roughly 93 x 33 cm at 300 DPI, and took at least a couple dozen hours to complete - including messing up and re-drawings parts of it.