Decide on your needs. Figure out whether you want a dehumidifier for your entire house or just one room, and how much moisture you wish to remove from the air.
Figure out the square footage of the area you plan to dehumidify. To do this, you will need to multiply your width by the length of your space. See the Resources below for a link to a square footage calculator.
Consult the Association of Home Appliance Manufacturers (AHAM) Directory of Certified Products for dehumidifier selection. See Resources for a link. Based on your square footage and dampness of your space, you will know the dehumidification value. It will tell you how many pints per 24 hours need to be removed. Use this amount when choosing your dehumidifier.
Check the AHAM Directory of Certified Products or check the dehumidifier box to see how many pints per 24 hours a product you've selected will remove. You will need to compare this with the number you got in Step 3.
Choose the more powerful one if you're unsure or stuck between dehumidifiers.