The way that I do my scale bars is to decide how much distance 1 pixel represents. On a world map it's like 2 to 5 miles per pixel. On a continent it's like 1 to 2 miles. Regional maps are like 1/4 to 1/2 mile. Building maps are anywhere from 10 feet to 3 inches. Once I have that estimated I grab the hard square 10 pixel pencil and plop down a square. Duplicate this layer, hold down the shift key and hit the right arrow to move this layer 10 pixels to the right. Do that 3 more times then merge these 5 layers together...this results in a 10 by 50 rectangle. If it fits my scale close enough I put a black color overlay on it. Duplicate this layer and move it to the right and change the black color overlay to white. Duplicate a few more times, alternating between black and white, then merge again. Put on a black stroke and then put the numbers in. If the scale doesn't fit my estimate I change my estimate