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.
Wait at least two weeks before washing the newly painted surface. After that, clean with any mild household detergent and a soft cloth or sponge.