Distance selling only obligates themechanism to accept returns in 7 days.
As for exchange they’d be under no obligation as mentioned above.
Bear in mind also the different size might well have been a higher price in the first instance too and whilst in my experience most places will honour a like for like swap when they are/were the same price I’ve had to pay the difference when the size I bought was/is on sale at a lower price than the one or turns out I need.
In practice i’d expect a lot of Web sales only companies to simply refund you and tell you to reorder [edit: as it’s simpler than an exchange], regardless of different pricing, so you’ll be in pocket of the replacement is cheaper and out of its not