What skateboard size should I get?

That is hard to determine for someone else because it is a personal preference. But there are some factors why a narrower or wider deck may be more relaxing for you.

The size of your foot can influence your deck choices.  Since for smaller feet, you don't want your deck to be too big. That can make certain tricks more difficult.

We recommend if your foot size is around 42 to get an 8.0 deck.  If you have a smaller foot size  around 37, I would go for 7.5. At a shoe size 45+, I would consider going for an 8.5. (It's normal if your feet are wider than the deck itself)  (42, 37, 45+ EU sizing)

For children up to 12 years, there are also special children's skateboards. These are smaller and are called Mini or micro 

What we recommend here is a suggestion. Nothing is wrong! It is mostly about personal preference.