set html text color and size using javascript

var elem = document.getElementById("MonitorInformation");
elem.innerHTML = "Setting different HTML content";
elem.style.color = "Red";
elem.style.fontSize = "large";

var myDiv = document.getElementById("MonitorInformation");
myDiv.style.fontSize = "11px";
myDiv.style.color = "blue";

Take a look at the JavaScript Style Attributes

I've abstracted a few methods, which could make them a little more useful for multiple invocations:

var el = document.getElementById("MonitorInformation");

function text( el, str ) {
    if ( el.textContent ) {
         el.textContent = str;
    } else {
         el.innerText = str;
    }
}

function size ( el, str ) {
     el.style.fontSize = str;
}

function color ( el, str ) {
     el.style.color = str;
}

size( el, '11px') 
color( el, 'red' )
text(el, 'Hello World')

Note: The best practice to dynamically change this type of stuff would be by setting the styles in a seperate external selector:

.message { color:red; font-size:1.1em; }

And toggling the class name, .className+= 'message' ( or an abstracted function to add/remove classes ).