All Alien arm sizes are compatible with all Alien body kit sizes. 
When selecting your Alien screw pack you must select the screw pack that matches the same size body kit you are purchasing. 

Included

• Alien 5" & 6" Full Screw Pack
• Alien 5" Arm Set - (4 Pack)
• Alien 5" Body Kit
• Blue PDB and XT60

Full build videos:

https://www.youtube.com/playlist?list=PLpY5rDaji8aysAopVocmxxwuJ6Bqn7LJv

Basic photo build guide here

http://www.rcgroups.com/forums/showpost.php?p=32330481&postcount=3

Info Minisite -  http://aliens.impulserc.com/

Individual Parts Weight (5" Body and 5" Arms)

• All Carbon - 91.7g
• Lipo Pad - 3.7g
• All Hardware - 25.5g
• All Motor Hardware - 5.2g

Complete Frame Weight  (5" Body and 5" Arms)

• 144.3g

Main portal for information on the frames is this thread...

http://www.rcgroups.com/forums/showthread.php?t=2473157

(Camera, LED Ring, motors, props etc pictured not included) 

Clips from FinalGlideAus & MrSteele!