Open your page in the Google Chrome web browser, rightclick any portion of the jPlayer on your page, and choose inspect element, in the debug window you'll see the HTML mark-up, hover or click the elements and you should see them highlight on the actual page, when you click an element, css styling information will display in the right sidebar of the debug window, here you can see widths, heights, paddings, margins, etc and most importantly what selectors, files, and lines where the styling is defined.
Opening the blue monday theme CSS file you can also start by looking for .jp-audio (should be at the top) and setting its width smaller (i think its 400px). Just to get you started. Good luck.