Wow... almost August 2017 and this issue (VERY basic functionality) is still a major problem. For anyone who's reading this post of mine, you have to: 1) make a marquee selection, 2) display the transform panel "Xfm" (sounds like a radio station), 3) drag the marquee around a little (you can't just click... you actually have to reposition the marquee), 4) and then you are able to enter numerical values for the size of the selection like you'd be able to do in Photoshop from the very instant you created a marquee selection.
As far as the Info panel is concerned... LOL. This useless panel simply doesn't give any info about your selection whatsoever. As is reported above, the values disappear the instant you release the mouse button, even if your marquee is still visible. Yet somehow, once out of every 20 times, the values do persist after releasing the mouse button. These issues have existed since April 2016 (when I bought the app), and continue today in version 1.5.2. Honestly I don't see how version 1.0 was released with this behavior, and why something so simple is still a problem over year later.