===== Antenna Lengths =====

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 \\

Online calculator 

http://www.onlineconversion.com/frequency_wavelength.htm