This is what you'll need to make a giant toothbrush:
Foam sheathing, acrylic paint, paint brushes, a hot glue gun, a surface smoothing tool (like a cheese grater), an exacto knife, a straight edge, handheld scrubbing brushes, a sharpie, and a pencil

Step 1: Outlining the Toothbrush

Measure and trace the outline of the body and neck of the toothbrush with a strait edge\yard stick.

Step 2: Cutting Out the Toothbrush

Use the exact knife to cut out the toothbrush, and use the straightedge again to make sure you are cutting in a straight line.

Step 3: 2nd Layer

To add to the thickness of the toothbrush, use the part you just cut out and trace it onto the foam sheathing and cut out another layer.

Step 4: Glueing Layers Together

After you've cut out however many layers you're using to make the toothbrush, glue them all together with the hot glue gun.

Step 5: Smoothing Out the Sides

Use a grater or brush to make the sides even, smooth, and more neat.

Step 6: Paint It

Use a nice size paint brush (not too big and not too small) to paint the toothbrush with acrylic paint. Don't use anything other than acrylic paint, like spray paint, to paint the foam sheathing or else it might melt.

Step 7: Decorate It!

This step is optional but if you want to make your toothbrush even cooler, draw something fun on it.

Step 8: The Head of the Toothbrush

Outline a square/rectangle around the scrubbing brushes, which will be used as the bristles, to make an outline for the head of the toothbrush and repeat steps 3, 4, 5, and 6.

Step 9: Putting It Together

Drill a whole into the neck of the body of the toothbrush, and into the head. Make sure the hole is centered and deep enough so you can stick a pencil (sharpened on both sides) into both of the holes so that the two parts of the toothbrush are able to come together. Also use the hot glue gun and put glue into the two holes before inserting the pencil into the toothbrush and then use it after you've put the toothbrush in to make sure the two pieces are securely attached.