the harness holds a half of a baby diaper and should be changed every 2-4 hours, much like a human baby... I wouldn't leave the harness on for more than half a day, with regular changes.... And I also give my ducks a bath after they've had their harnesses on. If they have access to food and water while wearing the diaper harness, it does get poopier quicker... if they are in a play pen, or in my case, in my front room, they don't have access to food/water while downstairs, the diaper isn't as poopy.
I have harnesses for all size duckies! i range from xxs-xxl! Your chuckie looks like a magpie? he'd probably fall in the medium range. the one that is in these pictures is the x-small. My ducks wear a small for runners (longer back strap) and so would any other ducks falling in the 3-5lb range. xxl is for big geese, but i am still experiementing with the bigger sizes.