Limit the size of a product image upload (dimensions) set default limits

If you  trying to change default uploading image sizes like minimum size accept is 500x500 and the maximum size allowed is 1000x1000.

Need to check validations open the file in your magento folder app\code\core\Mage\Catalog\Helper\Image.php and search for  below function
public function validateUploadFile($filePath) { around the 636 line

public function validateUploadFile($filePath) {
            $maxDimension = Mage::getStoreConfig(self::XML_NODE_PRODUCT_MAX_DIMENSION);
            $imageInfo = getimagesize($filePath);
            if (!$imageInfo) {
                Mage::throwException($this->__('Disallowed file type.'));
           }
        if ($imageInfo[0] > $maxDimension || $imageInfo[1] > $maxDimension) {
            Mage::throwException($this->__('Disalollowed file format.'));
        }
        $_processor = new Varien_Image($filePath);
        return $_processor->getMimeType() !== null;
    }

Change to

public function validateUploadFile($filePath) {
  $maxDimension = Mage::getStoreConfig(self::XML_NODE_PRODUCT_MAX_DIMENSION);
  $imageInfo = getimagesize($filePath);
  $testsize[] = getimagesize($filePath);
  if (!(getimagesize($filePath)) || !(($testsize[0][0] >= 500 & $testsize[0][0] <= 1000) && ($testsize[0][1] >= 500 & $testsize[0][1] <= 1000))) {
     Mage::throwException($this->__('Image size must be same or greater than 900px * 900px .'));
  }
  if (!$imageInfo) {
   Mage::throwException($this->__('Disallowed file type.'));
  }
  if ($imageInfo[0] > $maxDimension || $imageInfo[1] > $maxDimension) {
   Mage::throwException($this->__('Disalollowed file format.'));
  }
  $_processor = new Varien_Image($filePath);
  return $_processor->getMimeType() !== null;
}