How do the rest of you choose a suitable one for the site you are going to be building?

Do you choose a domain for the niche you will be using or, do you choose the niche by the domain name?

Also, do you think there is an optimal amount of characters in the domains you register? For example, generally speaking, when i register domains i like to keep them as short as possible and, preferably below 15 characters in length.

Do you even think it matters overall when choosing domains about the length of them?

Regards,

Lee