how can i convert my blog layout to be full width (browser screen) pages

You would first need a Child Theme or a custom CSS plugin if you do not already have one, then try editing this code a bit and see what happens:

#page {
    margin: 2em auto;
    max-width: 1000px;
}

Nice, that’s works fine for me at 1200px.
thank you so much #leejosepho @leejosepho.
now i’m trying to adjust the slider width and showcase content area width.
would you help me to adjust the showcase content width ???
by the way; the rest of blog pages already automatically adjusted with the new width 1200px.
thank you again

now i’m trying to adjust the slider width and showcase content area width.

I just use Firefox to right-click on something and then look for its code, and that is how I had found your #page styling. In the case of the content width, I would guess you will be looking for values within something like this:

#content {

}