You do not have to have a measure 3 spaces. If there is only one chord per measure, you can leave it at 4 spaces. Check out waltzes in the exercises, or Take Five etc.
The main problem arises if you have other chords in a measure. The player will attempt to make some sense of it according to how many spaces are in a measure, where the other chords are placed, and the player style selected (swing, eighths) but will bring up a player error if it is uncertain.
The idea of 16 spaces across means the chord symbols will fit within the spaces and still be clear in small screens (phones).

Of course if you are not using the player feature, you can put anything anywhere it makes sense to. For complex tunes/time signatures, constructing exactly what you want might be better done in notation software instead, not in iReal Pro.