You need to set up the actual size for the button (36 x 36 pixels or what you need). Use the Layers panel to verify the size of the button (or it's largest part).
Then remove unnecessary blank space. Click the Set Size button in the main window. In the dialog that opens, click the Set Size to make the document size equal toy your design. If you wish to leave small blank border around, set its width in "Fit all objects using margins". To remove the border, there must be zero. Finally click OK.