please try using the Launcher app from the link in this post. The java-based installer has a bug calculating windows sizes on Retina displays, using the Launcher will circumvent this.
"Due to a conflict with the newer 4K and 5K monitor resolutions, a Java error can sometimes prevent The Elder Scrolls Online from installing properly. To resolve this, please try the following steps: Go into System Preferences and choose Displays. While holding down the [Option/Alt] key on the keyboard, click Scaled. This will display all available screen resolutions. Select 5120x2880 as the resolution. This will make things look a bit odd temporarily, but it will allow installation of the launcher. Once the launcher is open, switch the resolution back to normal. This process circumvents any upscaling to avoid the Java error that causes the installation to fail"