Bumper stickers and dealer emblems are usually attached to a car with glue. The best way to loosen glue is heat, so try heating it up with a hair dryer. Use a putty knife, with its blade wrapped in duct tape to work the label up as the glue warms up.
What size do you need? Many experienced carpenters use a 16 oz. claw hammer for most jobs, but also carry a 20 or 22 oz. framing hammer for their heavier nailing jobs requiring big spikes. For lighter work, a 13 oz. curved claw hammer can be useful. Even a 7 oz. hammer will come in handy for smaller jobs like making picture frames.