Amber, BB's fragrance calculator is located here: . That will tell you how much to use for fragrances you get from BB. To find the calculator again, there's a link to it on the left-hand side of every Brambleberry store page toward the bottom. So you can get to it from almost every page on their site. Email or phone your other suppliers if the information isn't on their site.
In terms of size of bottles, I buy small bottles (1 oz) for fragrances I haven't smelled or used before, so I can test if I like them. If I have a chance to smell an FO in the bottle, and like it, I'm willing to buy 4 or 8 oz to test it out. Once I know I like a fragrance, I normally pick up 16 oz bottles. So with an average usage rate of 0.6 oz per pound (I just made that up but it's probably close for me), how long they last really depends on how big of soap batches you make and how often.