There currently isn't a way to check how much data a local user's persistence data takes up in the PlayerData or PlayerObject data 100kb limits on the server. We could manually count the bytes, however, it's noted in the docs that VRChat uses a compression format meaning the final size is unknown compared to the raw byte size: https://creators.vrchat.com/worlds/udon/persistence/#limitations

Functions like and would help with worlds storing large sets of data by returning a byte count of the compressed file size for specifically the local player. This would also be better than manually counting due to other unknown extra information that may be appended on the data.