How do I make the icon bigger?

There may be different reasons. Either the icon is only containing smaller images or there is empty space around the edges. Both causes can be relatively easily fixed by either selecting the larger image formats when converting image to icon or by removing the empty borders from the Image menu.

How do I remove the empty borders?

There is a Remove empty border command in toolbar or in menu when an image is opened in the editor. It will only work if the background has a single color. If it is not the case, you'll need to use the Flood fill tool to erase the background manually or use the Crop tool to remove the border manually.