When a table is selected, there is a floating menu above it with options for adding/removing/merging rows and columns. You can also enable a header row to make the first row more distinct.

Tables only support basic features, compared to other apps like Microsoft Word or Google Docs. Tables are best used for displaying basic tabular data - for more advanced layouts, it would be better to create the table in Google Sheets/Excel Online, and then embed it in the newsletter using the generic embed feature.

One example is manually resizing the width of columns. Because the newsletter is read on multiple screen sizes (wide desktops to skinny phones), the columns are automatically sized according to the content in each column, and the size of the screen the newsletter is being viewed on. You can add some spaces to the end of a cell in a column to make it wider, however keep in mind that the layout will likely change when viewed on a phone/tablet.