Sparrow wallet shows you the transaction in hex. If you have the hex string, you should be able to divide the length of that hex string by 2 (since one byte basically fits into two hex characters: 0xff = 255 = 2^8 - 1 and one byte is 8 bits) to get how many bytes this transaction has.
However, since SegWit (Segregated Witness), we use "virtual bytes" for fee rates. So the witness part is weighted less. So it's actually more complicated than just counting bytes.
That's where my knowledge starts lacking, so I'll stop here. Hopefully someone else will chime in and explain it better to you in more detail.
0xff = 255 = 2^8 - 1and one byte is 8 bits) to get how many bytes this transaction has.