You will not have to make any modifications on the PowerStore to get different priced options to work.
When you are either adding a new product or editing an existing one, you will want to click on the add/edit Options button at the top if you are editing or check the add product options checkbox at the bottom if you are adding a new product. Once on that page you can add an option (for your example, something like size) and then add values (the different sizes). After you are done with this, you should click on the "Click Here To Manage Options" button to make the ones you want to be shown on the live web page live. You can also edit the stock here as well. To change the price of a specific option, you will have to go to the edit product page. With the option drop down select the one you want and change the price for it. If you want them all to be updated to the same price, you can click the update all options checkbox next to the Price box.