I don't know the size of the arrow you are trying to insert into the table but, you can try inserting an arrow via the INSERT ribbon in the "symbol" section on the far right. When you click "symbol" a drop-down menu will appear. If you cannot find the arrow, you can enter the Character Code of 2190 through 2195 from the UNICODE (hex) character set to select a right, left, up, down and a combo up/down arrow. If these arrows are too small, you can adjust the font size for the arrow to increase the size.