gerbil wheel size

My son is saving up to buy all the materials needed to have 2 gerbils. He really wants to buy an exercise wheel. I keep coming across warnings about the type of wheel to use but I'm not sure about the size since I haven't actually seen a gerbil up close. (No one has any around us at the moment - hopefully that changes when he's ready to buy). We were at the pet store today and saw one that was 5 3/4 inch diameter. Is that ok or too small? The next size up was drastically bigger. Thanks for any help!