Hi fsky,
Maybe you could give us the title of the paper then someone may find a clue.
a)For the dipole, I would try the radius 0.01 wavelength as it may be convenient for fabrication?
b)If the impedance matching is not good, it may not only caused by the dipole, you may need to refer to some fabry-perot antenna papers.
c)if the patten is not so directive, maybe the HIS or PRS is well designed. you may need to check the reflection coefficients using floquet mode in HFSS/CST