However there is trick to size the images as per your requirement (customizing image size) while uploading any image on blogger.
This is how you do it.
1. Upload your image on to your blog post as usual and then open the 'edit html' option to find the URL of the image.
2. Refer the above highlighted part on the URL, this is nothing but the pixel size of the image.
3. If you change the size of this URL, the size of the image to changes.. the size could be eg. s200 or s400 or s800 etc...
So customize the URL address to customize the image size.. Thats it...
Some sample images provided below, have a look