1- what aspect ratio?
2-what is the perfect size?
3-crt or others? and if i can't find a crt machine, what should i get instead and in what resolution?
4- portrait vs landscape is the best?
5-any tips i should know? like paint after/before putting the buttons or putting the vinyl graphics before building the arcade cabinet?
6-what resolution
The answers to all those questions are highly individual. No two people will answer them the same way.
For a new person getting started, most likely, you are going to want a 27 or 32" 16:9 LCD monitor for ease of purchase and use. That will be large enough to make all games look good at horizontal or vertical orientations.
That said, if you have a deep connection to old school gaming, and want your games to look arcade correct, and have the money and time to invest in learning about them, a CRT may be the way to go. On the other hand, if you had to ask those questions, you are probably not the type to go the CRT route.