Tom's Life Bits

A simple monopole antenna should work fine.
Many suggest for 433Mhz, you can use #20 guage wire of length 17.3cm (6.81 in).

Did an internet search just now, and found on jeelabs forum, a list of calculated values–which look reasonable:
433 1/4 wave = 164.7mm
433 1/2 wave = 329.4mm
433 full wave = 692.7mm

868 1/4 wave = 82.2mm
868 1/2 wave = 164.3mm
868 full wave = 345.5mm

915 1/4 wave = 77.9mm
915 1/2 wave = 155.9mm
915 full wave = 327.8mm