S1 sends fund to R1 using a stealth addresses.
"stealth address" is an unfortunate moniker. When a user sends funds to another users address, they create new, special, outputs that the receiver now owns and only the receiver can spend; the receivers address never gets stored on the blockchain, only these newly generated outputs, which have no observable linkage to the receivers actual address. "stealth outputs" probably better describes them. Anyway, I digress...
The anonymity set (size) for the receiver (R1) is the size of all possible receivers in the blockchain.
Once R1 spends the funds, the anonymity set (size) for the R1 is 11 (size of Monero ring signature).
Sort of. It's not so much the spenders anonymity set is 11 (the ring size), rather the true output being spent has an anonymity set size of 11. For each input (an output) being spent, is one of 11 possibles.