Regarding Hiking Boots: We make every effort to be very clear about shoe sizing on our rentals page.
Regarding rental shells: our Men's and Women's shell jackets are U.S. Mountain Hardwear jackets. You can find a size chart for Mountain Hardwear jackets here.
Despite our greatest efforts, sometimes the size that's been ordered simply doesn't fit due to differences in sizing expectations. In this event we'll try our hardest to replace the rented item with another item that fits, but sometimes that just isn't possible due to limited inventory. This is a risk associated with renting any type of hiking gear. The least-risky option you could choose is to buy your own gear.