The size of the pitch allows you to control the area each team has to work with.
A wider pitch means defenders have larger with to cover and lends itself to aggressive teams that like to cross the ball.
A narrow pitch is effective when wide men are not being utilised as the main attacking threat: ACM'd can excell on a narrow pitch.
Long pitches mean there is a bigger gap between each line and "passing into space" is absolutely lethal if used by a faster team since they can cover the space faster.
A short pitch condensed the lines making for a compact defence (harder to pass through) but it's easy to play long ball on since the Target man can be reached by a diagonal ball in from the full backs in their own half.
When choosing your pitch size, don't just factor in your strengths. Also think about how your pitch can be used against you.
eg. Don't have tall or strong defenders? DON'T use a shot Pitch!!!
These are just examples but there are hundreds of ways pitch size influences the teams on them.