It's not possible right now. There are some predefined zoom levels that are used as zoom steps. Could you send some screenshots to show what would you like to achieve?
One thing that you can try is to maximize the editor by double-clicking on its tab. This way you'll get more editor space which might be helpful.
JK16 Jan 2017
I have a reference / model app running in the mobile. I take screenshot from the mobile and copy it to PC. I know that in the reference app certain buttons are large Bootstrap buttons. Now when I have this reference / model screenshot and the WireframeSketcher side by side I would like to scale WireframeSketcher screen so that large Bootstrap buttons are about the same size as in the reference screenshot. Font size ofcourse also must be same in screenshot and WireframeSketcher. This is not possible now because zooming steps are so large that buttons are either too small or too big.
Wouldn't it be more straightforward to paste your image directly into a screen and use it in the background? This way it could serve as a reference and you could draw directly over it. You can also resize the image so that its scale matches that of WireframeSketcher. Later you can remove the image or mark it as an annotation so that it doesn't get exported (with Hide Annotations option checked).