There really isn't any difference in sizes between males and females. It really depends on the genetics. A german giant male (or female for that matter) would be bigger than a regular bearded dragon, mostly due to the fact german giants are bred mostly for size, as oppose to color.
I don't think there is much difference in teh personalities of males or female bearded dragons. However other breeders have indicated females tend to be a bit more layed back, but others say males are.
Each dragon is different, some may be so layed back getting them to even flare out their beard is virtually impossible, others flare it out at every single movement no matter what you do to team them down. Fortunately the majority of dragons fall inbetween the two extremes, leaning more towards the calm. With patiences and persistance, I suspect 90% of dragons will become used to being handled and will sit quietly on a lap or stomach (if lying down).
Bearded dragons can and do bite, but their bite isn't very powerful. I have been bitten by my bearded dragon in the past, only one time she broke the skin and that was only because she managed to get her teeth on a spot of tender flesh. Most times they just scratch the surface of your skin (ie the predominantly dead part of your skin) and put a bit of pressure. This is with an adult dragon too, babies don't even scratch the skin, much less put much pressure down.
Defensively, dragons will flare out their throats (hence 'bearded dragon') darken it, gape their mouths, flatten their bodies, hiss and even lunge towards you. Very rarely do they actually bite you in all this bluff. Unless you happen to get a particularly mean one. Just don't let all the bluff scare you and the dragon eventually stops doing it.
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PHLdyPayne