Hi maxrus,

It sounds so simple that no one has found it necessary to answer you! Indeed, we can easily retrieve the height and the width of a page, then calculate the diagonal, then create a square based on that length.

Anyway, let's try to make such routine work in any context, supporting any custom measurement units, rulers settings, rotated spreads and/or even scaled/skewed pages! To do so we need to avoid usual methods based on "geometric bounds". Interesting challenge!

Here is my attempt:

// Create a square based on the active page's diagonal length (10%)
// ===========================
 
function measureDiagonal(/*Page*/page)
// -------------------------------------
// Ret. the page's diagonal in pts (relative to the page CS)
{
    var CS_INNER = CoordinateSpaces.innerCoordinates;
    
    var wh = page.resolve(AnchorPoint.bottomRightAnchor, CS_INNER)[0],
        w = wh[0],
        h = wh[1];
 
    return Math.sqrt(w*w + h*h); // Pythagorean theorem
}
 
function createTopLeftCornerRectangle(/*Page*/page, /*num[2]*/wh)
// -------------------------------------
// wh: width and height of the rectangle in pts (relative to the page CS)
{
    // Some const shortcuts
    // ---
    var    CS_SPREAD = CoordinateSpaces.spreadCoordinates,
        CS_INNER = CoordinateSpaces.innerCoordinates,
        RM_REPLACE = ResizeMethods.replacingCurrentDimensionsWith,
        AP_TOP_LEFT = AnchorPoint.topLeftAnchor;
 
    var spread = page.parent,
        // Create a rectangle (in the spread CS--the page CS is not relevant yet)
        // ---
        rec = spread.rectangles.add({fillColor:'Black'}),
        // Page transformation values (relative to the spread)
        // ---
        pageMxValues = page.transformValuesOf(CS_SPREAD)[0].matrixValues;
        
 
    // Normalize the rectangle in the spread
    // (the size does not matter here)
    // ---
    rec.reframe(CS_SPREAD, [[0,0],[10,10]]);
    
    // Apply the page transfo to the rectangle
    // so that its inner space fits the page space
    // ---
    rec.transform(CS_SPREAD, [[0,0], CS_SPREAD], pageMxValues);
    
    // Finally, resize the rec
    // ---
    rec.resize(CS_INNER, AP_TOP_LEFT, RM_REPLACE, wh.concat(CS_INNER));
    
    return rec;
}
 
var    FACTOR = .1, // 10%
    win = app.layoutWindows.length && app.activeWindow,
    page = win && (win instanceof LayoutWindow) && win.activePage,
    size = page && page.isValid && FACTOR*measureDiagonal(page);
    
 
size && createTopLeftCornerRectangle(page, [size,size]);

Not sure this is exactly what you were looking after.

Anyway...

@+

Marc