The White Matte (XW) is an uncoated paper-based material so it is not recommended for applications that expose it to moisture. For refrigerated packaging, we'd suggest a weatherproof material like our Weatherproof White Matte (JW) or our Weatherproof White Gloss (IW). The dimensions are listed as width x height, so the difference between the 3" x 6" and 6" x 3" would be the orientation. You'll want to verify the minimum and maximum widths of your label applicator to determine the ideal orientation. You're welcome to use our Maestro Label Designer to design and print your labels. This software, like many others, does allow you to rotate your design if you need to change the orientation. https://www.onlinelabels.com/maestro-label-designer