You can fix the max size of IR rendering in the Preferences section:
I know I'm a little late to the topic but anyways...
Setting a max. resolution for IR is only a work-around and should not be seen as the final solution IMHO.
For example the way I work is that I have a render-setting with small resolution to test-render the whole image, but if I want to check a texture or some detail with full final resolution I switch to my final-setting which renders to 5000x5000px. The IR window then blows up across both my screens and has to be manually resized every time.
In a perfect world we should be able to resize the CVFB however big or small we want.
The minimum width is currently restricted by the icons in the top row which can't be modified/deleted - even if I have never used some of them in years I can't get rid of them.
Also maybe add a feature where you could drag an area in the viewport which is then sent as render-region to the IR.