update code size to 3+1 bytes

EIPS/eip-6800.md

@@ -167,11 +167,13 @@ An account's `version`, `balance`, `nonce` and `code_size` fields are packed in
 | Name        | Offset | Size |
 | ----------- | ------ | ---- |
 | `version`   | 0      | 1    |
-| `nonce`     | 4      | 8    |
-| `code_size` | 12     | 4    |
+| `code_size` | 5      | 3    |
+| `nonce`     | 8      | 8    |
 | `balance`   | 16     | 16   |
 
-Bytes `1..3` are reserved for future use.
+Bytes `1..4` are reserved for future use.
+
+Note: the code size is stored on 3 bytes. To allow for an extension to 4 bytes without changing the account version is possible, reserved byte #4 should be allocated last.
 
 When any account header field is set, the `version` field is also set to zero. The `code_keccak` and `code_size` fields are set upon contract or EoA creation.