So first, I can't explain why your wife said it. I don't know if they are small compared to men that she's been with. You'd definitely have to ask her that.
However, there is a wide variety of "normal" for testicle size, and if you are concerned that yours are small, have your doctor take a look. They are measured in volume, so a doctor measures, not you.
Also, hanging low - that varies widely from man to man. Just like breasts vary widely from woman to woman, penises and scrotums vary widely.
I don't know why she said it - if she said it out of concern for your health, curiosity, or something else. I don't know if she's been with other men or how many there were, if she has.
I'm sure you're just fine, but talk to your doctor if you're concerned. If you've never had a doctor examine the area, it's not a bad idea to have it done, just so you know it's all good.