Two screen set up

I’m not sure what you mean by an image that is too big. The displayed window size exceeds what can be shown?

If the display resolutions cannot be determined, that can mean the iMac doesn’t generate the expected proper resolution for the external display, which might be a cable problem, or might be a problem with the display. Some displays can report only a subset of their actual display resolution capabilities, and macOS follows the display reports and Windows can ignore them and allow “unsupported” resolutions. Check with the display vendor’s technical documentation for the monitor.

Here? Reboot the iMac, and see if that clears whatever happened here.

If your iMac permits a newer macOS version (such as Mojave), also consider an upgrade. If you’re not sure about upgrades, post the iMac model year here, and somebody can post the available upgrade options.

What’s the monitor vendor and model of the display, what’s the current and the offered resolutions within System Preferences > Displays, if there’s any embedded firmware for the display from the display vendor then check for and apply any updates for that, and what’s the connection path (HDMI, DisplayPort, etc, and whatever dongles?) to the display?

A semi-ugly option continuing to use 32-bit apps pending replacement is running Mojave as a guest in a hypervisor.