I had the same problem for the past 30 minutes and tried the ideas above to no avail on a 2014 Macbook Air running Mojave. Ultimately, macuser_22's method worked:
"Finally, another option is to grab any portion of the window you can and hold down OPTION+SHIFT keys and drag, which will resize the window in any direction"
I fumbled around awhile but was able to grab the right hand side of the window and resize. Such a relief after all that work and being literally on the last step! Such a disappointing and frustrating end to what was a good experience using TurboTax this year.
