Do you want to determine the size of the data from the SaveData function?
If you want to determine the size of the data from the SaveData function, I afraid that there is no way to achieve your needs in PowerApps currently.
Currently, there is no doc mentioned about the Files limits of the SaveData function. Actually, the SaveData function is limited by the amount of available app memory because they operate on an in-memory collection. You should generally expect to have between 30 and 70 megabytes of available memory in your Mobile device.
All data sizes within PowerApps are subject to the amount of available memory in the app. Please refer to the following article for more details: