If this issue already exists, please let me know and I'll be glad to kill this one and vote on that one instead.
In the old editor, tables can be set to "Responsive" or "Fixed." The latter allows for specific column sizes and it's almost what is currently available in the new Fabric editor. I say almost because the old editor used fixed pixel dimensions for width while the new one uses percentage widths, perhaps in order to scale better on mobile devices. In any case, while the "pseudo-fixed" nature of new column widths can be frustrating for many, it is not the issue I want to focus on here.
The Responsive version of tables in the old editor allowed for completely dynamic column widths. The columns expanded as necessary to accommodate their contents. Additionally, the table itself was only as wide as necessary to fit the contents of all the cells in all the columns. This is not currently possible in the new Fabric editor's version of tables, and it limits the usefulness of tables (and wastes significant time by requiring manual updates to column widths after updates to content).
I realize that responsive tables can be performance-degrading in browsers, but this really is a useful feature, especially in smaller tables, where performance isn't as much of an issue.
For more discussion on this, see my original post here: