Do you mean that you want to figure out a wardrobe to take to the US with you, or do you mean that you want buy the kind of clothes you describe when you get to the US? I will assume you mean the latter.

My favourite chain of stores for smart casual clothes and work outfits, which I always visit (several times) whenever I visit the US is Ann Taylor. Ann Taylor is the somewhat dressier, somewhat more expensive line (but still quite affordable). Ann Taylor Loft is somewhat more casual and less expensive, but still has work clothing as well as lots of weekend options. I don't know if sizes stocked in the shops go up to Size 16, but it's quite possible since their online catalogue goes up to 18. One thing in particular I like about Ann Taylor is that it's very easy to put the clothes together - the stores stock coordinated (but not too matchy-matchy) combinations, plus accessories. I'm not sure where you'll be going in the US and how much time you have for shopping, but you should also search for Premium Outlet Malls, which carry the Ann Taylor Factory line (very affordable basics). Have a look at www.anntaylor.com and www.anntaylorloft.com to see if you like the styles.

Without even leaving home, you could shop Landsend.com. They have a wide range of styles, lots of sizes (including petites and long lengths), lots of practical easy-care clothes and they deliver internationally at quite reasonable prices (from the UK).