I am 5'7" and ~155lb, what size should I get? I'm usually between a medium and large.

I am about that size as well. I take a large bottom and an extra large top. The bottom has elastic all around so it should fit you well. I like an extra large top because I like my tops loose rather than tightly fitted. I hope this is helpful for you.

May i return this item at any dickies store in sd?

I do not know the answer for this question. However, I believe that Amazon requires you to use them as your method of return on all purchases. Amazon return process is extremely easy. You just go on to your account. Select "My Orders." Then, select the order that contains this item you wish to return. You will have several options to choose for your return, including methods for your refund. They even have a way that you will not need to print any label. Each step of the process is very well explained. If your item was a "Amazon Prime" item, you most likely will not have to pay for Return Shipping. I think Amazon's Return Process is just as easy... if not easier, than going to a physical store to make a return!

I normally wear a size 18-20 in pants. What size would I wear on your size chart?

I would purchase a 2X. I wear a size 22 and a 2X fits me. I would say a 2X fits size 20-22. Be sure to take into account petite and long options if you are under about 5’3” or over about 5’8”. This is just my personal opinion as I’ve worn these pants for years. They are my favorite! All my sisters and mom have taken to wearing these as everyday pants.