I don´t want to surf on that huge thing!
You are a beginner at surfing. First day going to the beach to try and catch some waves, you get this huge freaking surfboard to start with. You´re thinking ´what the …?!´, ´how can I ever surf with this monster?´. The bigger boards, 6 to 9 feet, are called fun boards. They have a lot of volume and are made to float. Exactly what you need to begin with to be able to let the board ´float´ on the waves so you can take your time to push yourself up on the board to be able to stand up.
These bigger boards are more difficult to take turns with for example, but you don´t have to try that when you are starting to learn how to surf. Let´s begin with just going straight ahead and being able to stand up on the board, or to catch the wave even when you are nervously holding on to the board trying not to lose on your way.
The bigger boards have more volume so it will be easier to balance on them (or: not fall off). They are called fun boards, because when the waves are small(er) surfers can have fun with those boards catching the smaller waves and playing around.
Because of all the volume it makes it hard to turn with those boards so whenever you are progressing and feel ready to make some turns or catch the green waves, it´s good to try a smaller board. Also when you are a little more tiny as a person, you can try to use a smaller board still with the same volume, like for example a 6.8 to 7.2 feet. As a small person it is better to carry and to survive in the waves.
So as a beginner: the bigger the better!