Sometimes the calendar widget is larger than your sidebars. Commonly this is because the theme’s stylesheet has added padding to all table cells. This can be countered by adding a rule that strips that padding back out of our mini calendar grids specifically.

.tribe-mini-calendar td,
.tribe-mini-calendar th {
padding: 0;
}

If you find that the mini calendar grids are still too big on iPad sized screens and would like to shrink it, you can reduce the font size with this:

@media (max-width: 850px) {
.tribe-mini-calendar {
font-size: 10px;
}
}

That will target the mini calendar grids font size after the screen size of the viewing device is below 850px in width. You can change that number to whatever size you would like. In addition, you may have to add more selectors to that CSS to get it to work with your theme.

Both of these can be added to the bottom of your theme’s stylesheet (we’ll leave it to you to decide if you want to do this in a child theme) or any other suitable place for custom CSS, including a custom tribe-events.css stylesheet or a plugin such as Simple Custom CSS.