I decided to take a photo showing 3 lengths of belly button rings. The first is your standard steel belly button ring, the second picture is of the 1.25 inch acrylic maternity belly button ring and the third, extra long, charm maternity belly button ring is the 2 inch length.
The metal ball, charm maternity belly button rings are very easy to rethread ( AKA shorten). If you can use a pair of scissors and screw a ball on, then you can shorten your Belly Charm with no problem at all. To keep it simple, most of these come in the standard, extra long, 2 inch length
I personally had to remove my steel belly ring and replace it with a maternity belly button ring around 5 months. I cut it down but did leave about 1/4 of an inch "growing" room. You always want these to have a little extra length so it does not fit tight. The main goal here is to keep the hole comfortable, and of course have a really cute belly ring at the same time.