How to change dimensions of a shapefile?

Hi, I have a .nc file with information on crop production. The file has a size of 720x360 (lat x lon) and a 0.5ºx0.5º degree of spatial resolution. I want to process this data by country and for this I need a worldmap shapefile with a polygons for each country. I am using the world borders dataset (acquired from: http://thematicmapping.org/downloads/world_borders.php). Once the dimensions between files are different, I cannot multiply them. Can someone provide me some guidelines? Thank you in advance, Teresa