There are a few things you'll need to consider when choosing the size of your foam runners. First, think about the width of your foot. Foam runners come in different widths, so you'll want to make sure you choose a size that's comfortable for your foot. Second, consider the length of your foot. Foam runners come in different lengths, so you'll want to make sure you choose a size that's comfortable for your foot. Finally, think about the height of your foam runner. Foam runners come in different heights, so you'll want to make sure you choose a size that's comfortable for your foot.