You can change the snapping grid's setting to match the size of your individual tiles. In the tileset editor, you have to first click the icon with a magnet and grid lines so it's turned on (blue color).
Then, a Snap Options category should appear in the Inspector. You can change your grid steps there.
You can also use the - and + buttons in the top-right of the tileset editor to control the image zoom and have an easier time framing tiles.