Fiducial design

Check this link for guidelines: http://www.accutroninc.com/pdf/download/fiducial.pdf

1mm diameter, round, with 2mm diameter of solder mask removed is the IPC guideline.

There's a bit more about it at http://denatechnologies.com/capabilities/pcb-fiducial-guidelines My impression is that various manufacturers may have slightly different preferences for the ideal one, so it's probably a good idea to find out if/what your manufacturer has a its own preference/guideline... assuming you're making the design for only one mfg. Dena tech recommends the R-3R (1mm/3mm) variant.

Accutronic in the document linked in the other answer also recommends the R-3R version rather than the R-2R [IPC] one, which they still support though.

The SMEMA std can be found here for example. Note that they say R-2R is the minimal design:

The size of the clearance area shall have a radius that is at least twice that of the fiducial mark and shall be concentric with the center of the fiducial mark

(emphasis mine).

The other answers are excellent. In addition, it is common practice to put a feature in the solder paste layer that matches the fiducial's copper layer. This is to assist aligning the solder stencil over the PCB.