I inherited and play my father's 4-1/3 octave Musser, made in about 1965. I would never attempt to reproduce this kind of Marimba.
Even larger marimbas are made by Marimba One, Yamaha, Vancore, Adams, and Musser. Some examples here:
http://inconcertpercussion.com/_wsn/page2.html
My guess is that you really can't make one of these without making it your career as a part of a highly trained team with specialized equipment. Theoretically, I could figure out how to build a concert piano, but not really effectively.
But that is just one definition of the word "Marimba".
One of the best experiences of my life was listening to two percussionists on an African Bush Marimba, which was bars of wood cut with an axe and a big knife, laid out on a bed of straw, tuned on a 5-note octave scale, about like the flats and sharps on a piano.
In-between is stuff that would be really fun, possible, and still a major tuning challenge. One of the marimbas that had a box for a resonator and up to two octaves of notes is in the range of the sort of possible.
The million dollar questions:
Are you courageous and challenged by projects just barely possible?
Have you ever tuned a guitar or piano or timpani or any other instrument?
If not, you need a partner who has done that. Either way, you need a chromatic tuner that can produce a tone and then tell you what note you are playing and whether you are sharp or flat. These are under $100 or can even be downloaded as a program that works through your computer.
If you cut all of the bars to perfect length and width and thickness, they will still be way out of tune. Each one needs highly skilled work. But isn't that what makes the challenge exciting. It's not easy. So what?
If you take on the right size challenge, go for it and have a wild time!
Brian Kent