Hi Mike. Thank you for your answer.
After the post, I did realize the window was blowing up my screen horizontally, and that's why I have those issues - at list the main reason. Your link lead to several topics covering a lot of problems I don't have and I though is strange only me having this issues, maybe only me is reporting, I don't know. I am using an old machine with low screen resolution, maybe with a higher resolution the windows will work properly. There is an old tip of zooming the window but this feature is not available in Spriter that don't follow macOS window style.
The solution so far is to drag the window to the left (the left side will be outside the screen), resize the window, drag it back to mach up left corner and adjust the bottom, right corner.
But there is a quick solution: fullscreen! No workspace issues in fullscreen.
About the resolution of my project, you're right, I just don't fix it yet because I am focused in learning all the steps in making a 2D animation in Unity, from the concept art to coding.
Finally, I can't complain about Mojave. Every new OS got issues, and Mojave has been very "peaceful" in comparison with predecessors Sierra, El Capitan, etc.. It is fast and stable - developers most wanted from a system.