You cannot edit the size because it's an SVG path. The size of those can only be edited within the tag and not with css. The only option could be with JavaScript. You have to edit the attribute d and figure out higher dimensions as path only uses dimensions instead of height and width.
Play a bit around with
path{
fill:red;
}
to see what you can do more with it.
This is the current tag from the left arrow:
<path d="M356,162L351,151L345,162Z" stroke="none" stroke-width="0" fill="#cccccc"></path>
Where d is the dimension.
Hope this helped you a bit though this is not the solution.