A Solidity snippet showing that `CODESIZE` returns the size of the called code (i.e. the implementation contract) when called via `DELEGATECALL`, rather than the size of the delegating contract.

Codesize.sol

// SPDX-License-Identifier: WTFPL
pragma solidity 0.8.19;

contract A {
    /**
     * @dev Returns 118 for contract `A`.
     */
    function codesize() external pure returns (uint256 code) {
        // solhint-disable-next-line no-inline-assembly
        assembly {
            code := codesize()
        }
    }
}

contract B {
    /**
     * @dev Returns 623 for contract `B`.
     */
    function codesize() external pure returns (uint256 code) {
        // solhint-disable-next-line no-inline-assembly
        assembly {
            code := codesize()
        }
    }

    function delegatecallCodesize() public returns (uint256) {
        A a = new A();
        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returnData) = address(a).delegatecall(abi.encodeWithSelector(a.codesize.selector));
        // `assert` is cooler than `require`, I'm sorry.
        assert(success);
        /**
         * @dev Returns 181 (and NOT 623).
         */
        return abi.decode(returnData, (uint256));
    }
}