Script to set artboard to selected art, etc.

Hey!

Here's an example that should do those three things, I've added some (pretty sparse) comments but let me know if you have questions about any parts in particular.

#target illustrator
function test(){
    // if there are no documents open, alert the user and exit
    if( app.documents.length == 0 ) {
        alert( "Please open a document!" );
        return;
    }
    
    // get a ref to the current document
    var doc = app.activeDocument;
    var artboard = doc.artboards[0];
    
    // flag for user input
    var validInput = false;
    
    var promptMessage = "Please enter the size of the rectangle in inches";
    var dims, rectWidth, rectHeight;
    
    // this section of code will be repeated until either of the two flags are set equal to true
    while( validInput == false ) {
    
        // prompt for the dimensions
        dims = prompt( promptMessage, "8.5 x 11" );
        
        // if the dims var is equal to null, assume the user clicked the cancel button and exit early
        if( dims === null ) {
            return;
        }
        
        // otherwise, assume the user clicked OK, try to parse the input
        else {
            
            // turn the string into an array, splitting on the letter 'x'
            dims = dims.split('x');
            
            // if there are only two elements in the array
            if( dims.length == 2 ) {
            
                // try to convert them to number values
                rectWidth = parseFloat( dims[0] );
                rectHeight = parseFloat( dims[1] );
                
                // if neither value is equal to NaN (not a number), then set the flag to let us out of the loop
                if( !isNaN( rectWidth ) && !isNaN( rectHeight ) )
                    validInput = true;
            }
        }
        
        // if we have to prompt again, update the message to have some more info
        promptMessage = "Please enter the size of the rectangle in inches\nValues should be in the format '<width> x <height>'"
    }
    
    // if we made it out of the loop, assume that the values are valid numbers!
    
    // convert the numbers to points and make sure they are both positive
    rectWidth = Math.abs( rectWidth * 72 );
    rectHeight = Math.abs( rectHeight * 72 );
    
    // get current artboard dimensions
    var bounds = artboard.artboardRect;
    
    // add new rectangle                                    top                   left                  width           height
    var rect = doc.pathItems.rectangle( bounds[0], bounds[1], rectWidth, rectHeight );
    artboard.artboardRect = rect.geometricBounds;
    
    // scale rectangle, numbers 
    rect.resize( 
            25,                // scale horizontal in %
            100,               // scale vertical in %
            true,              // change position
            false,             // change Fill Pattern
            false,             // change Fill Gradient
            false,             // change Stroke Pattern
            1,                 // change Line Widths
            Transformation.RIGHT );  // scale about which point
    
}
test(); 

This script has examples of properties and functions, as well as conditional statements and loops. All of the functions and properties, such as the 'documents' property of the App class and the 'rectangle' function of the PathItems class, should be listed in the "Scripting Reference: Javascript" PDF.

How experienced are you with JavaScript in general? If you are still relatively new, I would suggest digging into that first. It will make the AI-specific docs much more digestible. There are some good intro web resources like khan academy​, code academy​, etc. that help with basic syntax and structure.

Good luck, have a good weekend!