What should be ideal length of any Domain

Hello All,
Is there any specific guideline for considering length of any domain. Apart from making it brandable is there is any impact on optimization of website.

I often see many websites use numbers in their domain is it just for ensuring availability of domain or does it have any other benefit?

How to ensure a domain name is brand-able or not?

Is there also any requirement that we should register any domain at least for a certain amount of years?

Ideally 63 characters is the longest you should try to make your domain name. As you find that most visitiors do not want to type a long domain name before they can get the information. Click this link to Read More on more domain name don'ts and dos here