How can I include product dimensions in my listings?

I have tiny objects that I ship that weigh ounces and only use a box that is 8x8x8" and I have very large items that need a box that is 24" and weight 6 or 8 pounds. Only choosing one default box size either makes the customer pay way too much for their shipping (if I choose the largest sized box we use) and therefore, I am losing sales, or if I choose the smallest box, I get ripped off on shipping charges. If I list on etsy, eBay, Bonanza or anywhere else, I don’t have to worry about this issue. I hope I didn’t dream this whole thing! But now I cannot figure out how to add dimensions to my products or populate lists for different sized boxes. Please help!