How to make a page width of 100% (Protean) UNA 10?

Which style mix are us trying to update? You cannot change any of the defaults. If want to modify the Protean Light or Dark mixes for example, you will need to Add New Mix from the Protean > Styles area and tell it to duplicate from.... whichever one you want. Then you can edit that one and make the changes.   

I know that. I can't put 100% on my mix. Understands only in numbers, for example 1920 px.

How to make the site was 100% of the montor?

See the screenshot above. The site ignores my edits as a percentage.

I am currently using a custom mix from the Protean Light mix and I have mine set to 80% and it works just fine. Maybe you need to clear all the cache via the Studio > Dashboard > choose clear all cache from the dropdown arrow next to cache.