The size of your image will be the size of your base64 stringEdit: apologies for blatantly wrong info. Acording to this - no moreWikipedia article, no less.it will be a bit larger:
Very roughly, the final size of Base64-encoded binary data is equal to 1.37 times the original data size + 814 bytes (for headers).
You can use ContentLength property of the request to determine what the size is in bytes, although if you are uploading more then one image, it might be trickier.