Use the same temperature for the same recipe, even if you've changed the pan size. You'd have to do something really drastic to need to change the temperature.
Bake time is almost entirely dependent on depth of batter rather than size of pan. As long as batter depth is essentially the same, so is the bake time. For example, I have baked a cake recipe for a 9 inch round in two 6 inch rounds with the same bake time.
If you double a recipe originally for a 8x4 loaf pan and bake it in a 9 inch round, then you'll have pretty much the exact same depth of batter. (who says basic geometry isn't useful in our adult lives?) In this case, I'd recommend starting checking the cake for doneness at exactly the same bake time for the original cake. If you half a recipe originally for a 9 inch round and bake it in an 8x4 loaf pan, I might start checking it a few minutes early just to be careful.
More generally, if you are halving a recipe, find out the surface area of the recipe's pan and halve it. If you are doubling a recipe, find out the surface area of the recipe's pan and double it. How does this value compare to the surface area of the new pan you want to bake in? If it's a bit higher, you might need to add a few minutes to bake time. If it's a bit lower, you might need to subtract a few minutes from bake time. If it's wildly different, maybe consider using a different pan.
Also remember that a cake isn't done at a specific time. The time on the recipe is to get you in the ballpark to start checking for doneness, and a recipe that says 35 minutes is not going to be hurt by quickly opening the door at 25 minutes to see if the top still looks wet. If you're moving to a smaller pan or lower depth of batter, it probably makes sense to start checking things on the early side. I usually start checking any cake 5 minutes before the time on the recipe, so if I'm messing with pan sizes or something like that, I'll start checking 10 minutes early.
There are a lot of other clues about doneness than just time - for a butter cake these are usually if the top springs back when pressed, if it's pulling away from the sides of the pan, and if a toothpick inserted into the center comes back with just a few crumbs. Reports of butter type cakes falling because the oven door is opened too frequently are greatly exaggerated.