When do I know it is time for a bigger diaper size?

Tight at sticky tabs. A diaper is too small if a parent has to pull to get the diaper closed with the sticky tabs. Also, one can just try the next bigger size. If the next bigger size is not huge nor loose on the baby, then just go with the bigger one.
Time for a bigger... ...Diaper when the one you are using does not close properly, or starts chafing.