It is a terrible app, I wish Wix would focus on getting these basics right instead of all the other weird stuff they do like trying to force naff apps on your website, like getting customers to join you on the Wix app, which means their contact form gets sent to the website developer and not the business owner. Wtf. Grrr.
Here is a workaround I used after wasting a day looking for a solution.
The table master app is setting the column width automatically, so to set a fixed width I tried typing in a row with a number of - in each cell to control the width. This didn’t work as the app didn’t seem to recognise those, but * did.
So I worked out the optimal number of * for each column and entered those as the first row.
It doesn’t look perfect and of course if the number of characters in a cell exceeds the number of * it all falls apart, but I think in my situation it will work 99% of the time.