The length of the board relative to your weight determines to a large extent how much flex the board will yield. It's similar to polevaulting- the higher the weight threshold on the pole, the stiffer and more resilient it will be.
If you use a board that is longer, it will be more stable at high speeds (this is moot for park riding) and will be more difficult to flex. This affects ollies, wheelies, presses, take-off, landing and jibs. Pretty much everything park-related relies on the board being very flexible and forgiving. Not as much for big air, but certainly for jibs and pipe.
On my downhill/all-mountain board, I can't do a press for shit. My park board is only 5 cm shorter and I can tail block that sucker fairly well. The difference is very noticeable.
If you're set on riding park, be disciplined in the size of the board you get. Don't sacrafice a few cms (and in turn, your ability) to buy a board quickly or buy a board you really like. Find something similar that has your size- it just makes for a better riding experience and it's no inconvenience to you.