To get a resized post image in Magento 2 Blog you can use this code in your template files:

   $width = 100;
   $height = 150;
   $imageHelper = $this->helper(\Magefan\Blog\Helper\Image::class);
   src="<?php echo $imageHelper->init($_post->getFeaturedImg())->resize($width, $height); ?>"
   alt="<?php echo $block->escapeHtml($_post->getTitle()) ?>" />

If you don't want to use the height parameter then you can execute the resize function only with the first parameter, e.g.

<?php echo $imageHelper->init($_post->getFeaturedImg())->resize($width); ?>

Or if you use Magefan Blog version greater equal than v2.9.8, you can use this line (it will allow you to save the image width x height file proportion and not create a square image with white borders):

<?php echo $imageHelper->init($_post->getFeaturedImg())->resize($width, null, false); ?>