Buy Running Shoes to fit the Larger Foot:

MD Podiatrist 
“Someone training for a long-distance run such as a marathon can log 800 or more miles before a race,” says Dan Michaels, DPM. “With your feet enduring this kind of mileage, it’s essential to choose the appropriate footwear to maintain the health of your feet and prevent injury.”
More than two-thirds of the population has one foot that’s larger than the other. This rarely exceeds one-half size. However, this should be considered when buying new shoes. Always choose your size based on the fit of the larger foot, since the fit of the smaller foot can be easily adjusted by adding an insert or arch support. That’s why it’s important to try on both shoes. If a shoe fits properly, there should be a half-inch of space between your longest toe and the front of the shoe, leaving plenty of room to wiggle your toes.