For example, the original width of the image displayed below is 468 pixels(displayed in full width):
But in Blogger, even when LARGEST size is selected, displays it like this:
And the original image is shown only when the visitor clicks the image.
So, if you don't want to show those small images, rather wants to show images in their complete size, then follow this method.
Upload and place the image where-ever you want, in the post. Then click on the "Edit HTML" tab link. Where you can see the image code something like this:
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgDO0nlCGkhbDvq-qNkB51Q0itZvFNm-l5Ox2_l-j13MaGpdL66E6I_jeoa26kLAHkfKDMzAUCDliFTOARLuNdvr-7aNgHXFMkEVGx8FRTitdr49GtdVQNrgcFbmsIAjm-lM9yh8Sf9FgNb/s1600-h/Chick1.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgDO0nlCGkhbDvq-qNkB51Q0itZvFNm-l5Ox2_l-j13MaGpdL66E6I_jeoa26kLAHkfKDMzAUCDliFTOARLuNdvr-7aNgHXFMkEVGx8FRTitdr49GtdVQNrgcFbmsIAjm-lM9yh8Sf9FgNb/s320/Chick1.png" /></a>
Simply change the code in RED color, to s1600. That's it, now you have the image in full size, now if you want to control the width according to your own choice, then simply add this code where src="" ends (give a space): width:"XXX"