// Display as many blocks on the screen as will fit
var BLOCK_SIZE:Number = 1;
var BLOCK_BUFFER:uint = 3;
var blockSize:uint = this.inchesToPixels(BLOCK_SIZE);
var blockTotal:uint = blockSize + BLOCK_BUFFER;
var cols:uint = Math.floor(this.stage.stageWidth / blockTotal);
var rows:uint = Math.floor((this.stage.stageHeight) / blockTotal);
var blockXStart:uint = (this.stage.stageWidth - ((cols * blockSize) + ((cols - 1) * BLOCK_BUFFER))) / 2;
var blockX:uint = blockXStart;
var blockY:uint = ((this.stage.stageHeight) - ((rows * blockSize) + ((rows - 1) * BLOCK_BUFFER))) / 2;
for (var colIndex:uint = 0; colIndex < rows; ++colIndex) {
   for (var rowIndex:uint = 0; rowIndex < cols; ++rowIndex) {
      var block:Sprite = this.getBlock(blockSize);
      block.x = blockX;
      block.y = blockY;
      this.addChild(block);
      blockX += blockTotal;
   }
   blockY += blockTotal;
   blockX = blockXStart;
}

function getBlock(blockSize:uint):Sprite
{
   var block:Sprite = new Sprite();
   block.graphics.beginFill(0xAAC228);
   block.graphics.drawRect(0, 0, blockSize, blockSize);
   block.graphics.endFill();
   block.cacheAsBitmap = true;
   return block;
}

function inchesToPixels(inches:Number):uint
{
   return Math.round(Capabilities.screenDPI * inches);
}

http://www.adobe.com/devnet/flash/articles/authoring_for_multiple_screen_sizes.html

Taken from Christian Cantrell's Adobe article called 'Authoring mobile Flash content for multiple screen sizes'.

AS3 Fill Available Screen Area with Squares (Centred)

center

ActionScript 3