When a babies or toddlers diaper is fitting properly the diaper should fit to where you can place at least two fingers easily into the waistband of the diaper and the diaper should leave no marks on the babies waist or around the legs.
If you can barely get the diaper taped on the baby or toddler without stretching the tapes or Velcro then the diaper is too small.
The diapers tabs should reach to the middle of the diaper or at least a few inches or two on each side of the diaper.
If the baby or toddler is leaking out the diaper then the diaper may also be too small to absorb the amount or urine the child is urinating.
A diaper that is sagging or cannot be taped on snug enough may also be too big but sometimes a diaper can be too small and too tight.