First, how does the top bar hive differ from other hives in terms of pest problems and pest control?
Top bar hives are not significantly different. Some say that the top bar hive is less stressful to the bees, making them healthier and stronger. That is not necessarily true. Stress does make the bees weaker, but there is nothing inherent in other types of hives that causes stress. Pest control is much the same in Top Bar hives as in any other hive type. You have to respond appropriately to each type of pest. Most treatments don't care what type of hive it is.
Second, I have seen a lot of conflicting opinions on entrances, bottoms, and ventilation.
First ventilation, because that is related to the other two. Ventilation is really important. Without ventilation your bees will not survive very well. You have to plan your hives with ventilation in mind. You can provide ventilation and let the bees control how much they want. They will propolize anything beyond what they need.
I have hives with bottom entrances and top entrances. Both work fine. I don't like entrances in the middle of my top bar hives, they require way too much manipulation and can effectively cut the hive in half, causing overwintering problems where bees can't get to the stores on one end of the hive or the other. I like top entrances because it enhances the ventilation in the hive. The hot air that naturally rises to the top has a way to escape. This is true for both top bar and langstroth hives. I don't have to cut any holes in the hive, making them easy to construct. Top entrances also help to keep out mice and help to deter skunks and possums.
Bottom boards are an interesting issue. If you want to use powdered sugar as part of your mite control, screened bottom boards are a must. However, my experience is that the bees are more likely to abscond from a screened bottom board top bar hive. I have heard that from others too. I lost two packages when I had a screened bottom board on my hive. Since I went to solid boards, I haven't lost any to absconding. Of course you can build a screened bottom board with another moveable board below it to control ventilation, which might be a good idea too. This is what I would use if I was treating for mites. You want the mites to fall through the screen so they can't get back up to the comb.
In my area we use solid bottom boards and don't have a big mite problem, at least so far.
Top bar hives present problems for some types of feeding. Some of my hives have holes cut into the bottoms to allow boardman feeders to be used in the ends of the hives. I have also made devices to allow feeding of patties that slip between the top bars, but you have to have a raised top to allow that.
Here is a link to some hive plans that are used in my area.
http://abqbeeks.org/forum/topics/tbh-plans. TJ is a very experienced beekeeper. We like to use the same dimensions for the hives so beekeepers around the area can share bars if need arises. This brings me to the point of suggesting that you should get in touch with local top bar beekeepers and see what they do. Conditions are different in each area, and what works well for me in the middle of the desert might be completely different from what works in Georgia. One thing I would suggest, however is that you don't make the hive too deep. When it is very hot, the wax gets very soft and collapses under the weight of the honey if the hive is too deep. I think you might have occasional hot days there in Georgia.
Third, most of the plans that I have seen, including the one that I have chosen, are approximately 44 inches in length. Aside from the awkwardness of it, is there any reason that I couldn't build one larger? 8 - 10 - 12 feet long?
I make my hives 42.5 inches long (inside dimension) because it is easy to cover them with 48 inch stock and 42 is divisible by 1.5, which is how wide I make my bars. The extra half inch is for the entrance. Up to 48 inches long is good. Most of my colonies didn't fill up the 42 inches this year. I have heard that the bees won't use much past 48 inches. The queen pheromone starts getting weak at that point. You could build a really long one like you describe if you wanted to put multiple colonies in it separated by follower boards, but it seems like it would be kind of a hassle, and it is way to big for a single colony.
Check out the top bar forum here. There is lots of information there. Also, Michael Bush has lots of great information on his web site,
http://www.bushfarms.com/bees.htm.