there are 2 factors influencing body guard size as already stated in this thread. high influence will always increase body guard size, as can be seen in any camp game u play, high influence gens will always have slightly bigger body guards. the second is personal security. i have seen 2 cases where this was show to be true.

first i should note i play on second biggest unit setting (cant remember wot it is) so my gens normally consist of about 20-30, faction leaders and heirs 40-50 depending.

after the romans repeatedly tried to assasinate several of my thracian generals in italy, i noticed i had 2 gens with low influence, who were not heirs or leaders with body guards of over 40 men! upon futher inspection i found that they both had large bonous's to personal security due to so many failed assasination attempts.

The second was when i saw a brutii faction heir, he had 0 personal security traits, and had a standard body guard of about 40ish. then i assasinated his father, and attempted to assasinate him several times. by the time i had lost 3-4 good assasins, he had 3 ancillaries which improved his personal security, 3-4 traits which improved his personal security and a body guard of 60 men!!!!
needless to say he lived to the ripe old age of about 30 till i finially assasinated him :D :D

Cheers Knoddy