Each square covered gets 1 point for your side, and removes a possible place for the opponent to play on.
Bigger pieces (4 and 5 squares) therefore are, in general, better to play than smaller (1, 2, or 3 squares), given the choice. This is especially true in the early stages of the game.
Furthermore, as the board fills up, places where the bigger pieces can play become limited, and the choices fewer.
You have a tool box full of tools - consider all of them, and choose the best one.
Look at the Red i3 in this picture. It's pretty clear what Red had in mind: Red wanted to ``hit the gap'' (see point 12 (bonus) for more information about hitting the gap), to have access to the higher part of the board.
That's a great idea, but what probably happened was that the Red player saw that the i3 accomplished this task, and thought to him/herself: ``I guess I need to use a 3-piece'', and just played it.
But it's way too early to be playing an i3, and the key mistake to avoid is forgetting to look for upgrades.
After the Red player sees that the i3 is a good move, he/she should look for better moves that ``contain'' the i3, or even better, look for better moves that accomplish the same goal as the i3.
(The i3 is also bad here because it blocked itself from going anywhere
to the right.)
In this picture, we can see a much better move that contains the i3.
It covers all the squares the i3 did - PLUS two more,
and it is not cut off on the right side.
and in this next picture is a move that accomplishes the same goal as the i3, but is a 5-piece, and is also better because it doesn't block Blue! Remember, self-blocking (including self-``blocking-to-a-1-hole'') is bad!
Don't do your opponent any favours by blocking yourself!