The number of rows is not by the number of elements

The number of rows of the categories of events was the same as the number of categories.
Please do:
1) element “size” must automatically accept the number of categories not number.
<select name="event_categories[]" multiple="" size="10">
screenshot
or
2) You can also just remove multiple="" <strong>size="10"</strong> and leave only <select name=”event_categories[]”>
Then the compact convenient list is obtained, which will use well on mobile devices.
That’s how it will look like – short list
Give users the ability to select the display method