Not all TX ratsnakes are aggressive; I know a seller here in central Ohio who has the calmest TX rats I've ever seen. Personally, I really like the little guys.
Truthfully, my recommendation would be to find a reptile show or sale in your area and check out some of the snakes IN PERSON. Believe me, it will give you a much better idea of what you like and don't like, and you'll be able to handle the animals to get an idea of their size and girth (i.e., the difference between a 5-foot blood python and a 5-foot ratsnake). There's only so much research you can do online, and it's hard to visualize snake lengths without seeing them firsthand.
Kingsnake has a great "Events" page that can show you events coming up in your area:
http://www.kingsnake.com/events.html
I strongly encourage you to attend any upcoming shows in your city!
Anyway, that aside, I would vouch for a Brazilian rainbow boa, although not without reservations. They are not the easiest snake to keep (due to their humidity requirements), but I think that if you can master the humidity, they might work out well for you. They max out at 5-6 feet for males, and 6-7 feet for females, and are more slender than the boa constrictors.
As Drosera mentioned, they can be a little nippy, but it's nothing to worry about at all - I've been tagged by an adult male Brazilian rainbow, and I didn't even realize he had actually bitten me until I saw a few pinpricks of blood. Honestly, getting bitten by my cat hurts a LOT worse. Snake bites aren't as bad as the media would like us to believe.
In any case, with regular gentle handling, a rainbow boa would probably calm down nicely - although not "bite-proof". To answer your question, most snakes will grow up to be trustworthy adults if handled properly from hatchling size. But some will go through a nippy phase as babies, like the rainbow boa. Research is the key here.
Anyway, I hope I didn't sound too overbearing or anything; I'm a newbie in this myself! But I remember when I was here asking the same questions over a year ago, and without the help of other users I wouldn't have learned so much about these great animals. Good luck in your search!
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::i believe in joy > http://www.winds.org/nekomi/hope.html
::my homepage > http://www.winds.org/nekomi
My Growing Zoo (Herps coming soon!):
1.0 Husband (Byron) ^_^
0.1 black cat (Shade)
1.2 Cockatoo cichlids (A. cacatuoides yellow-gold)
1.1 WC Cockatoo cichlids (A. cacatuoides blue Peru)
3.3 Pygmy corydoras (C. pygmaeus)
2.0 Endlers' Livebearers (P. sp. Endlers)