What discipline do you want to do in skateboarding?

What discipline you are going to choose can affect your deck choices. 

- Street, park 

Usually between 7.5 to 8.5 

- Pool, mini, vert

For this, you usually want to go a little bigger than what you would normally go for. 8.5 and above or old school pool decks 

- Freestyle

For this you often see that they have some smaller decks usually between 7.25 - 8 besides that they are also usually a bit shorter in length (You also have specials old school deck for this)

It's really about personal preferences. There is no wrong choices you can make.