I wear a size 10 in most boots or shoes and I ordered the medium. With insoles installed, these fit great and I walked a day at a Faire on moderately hilly, uneven ground for most of a day with no issues.  I have somewhat flat feet so I couldn't really comment on arch support. While I won't be expecting to be able to use these for hiking or daily wear, I think they will be great for occasional cosplay use.