Yeah, when you add the image as an attachment and then click 'full image', once it loads into the post you can click it and then the corners get little widgets on them that you can grab and resize the image. For ease of browsing i've found the max size should be about half the width of the post box
or this:
(not going to resize this one as it's specifically demonstrative.)
Of course, some measure of flexibility is necessary as some images are very wide but quite short and shrinking them to that size makes them illegible so it's only a rule of thumb.