first switch to "edit html" mode on your post editor and find the image you would like to adjust here's my sample image code
<a href="http://1.bp.blogspot.com/_JgEGJbRLYj4/SZhUSysfZJI/AAAAAAAABG8/XkkheuA60xU/s400/step-01.jpg"><img style="width:304px;height:400px;" src="http://1.bp.blogspot.com/_JgEGJbRLYj4/SZhUSysfZJI/AAAAAAAABG8/XkkheuA60xU/s400/step-01.jpg" border="0" alt="" id="BLOGGER_PHOTO_ID_5303081243051517074" /></a>
yours may look a bit different from mine but don't worry we are only interested in the code highlighted in red this controls the width and height of your image.
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