how calculate plaintext file size from length of encrypted one without decrypting
As far as I understand ,all original files with length between let’s say 33 and 47 will have length 48 bytes after aes-128-cbc encryption (should be rounded to next 16 byte block).

Actually, if you are using the standard padding pattern (and you don't count the space the IV takes up - you do use a different IV for each ciphertext, don't you), then the range of original plaintext strings will be 32-47; those will all encrypt to 48 bytes, and no one without the key can determine the original length.
However, if you do have the key, you can determine the length without decrypting the entire message; you only need to decrypt the last block - that is sufficient to determine the original length.
That last block is $P_n = C_{n-1} \oplus D_{key}( C_n )$.
For a 3 block message, the savings aren't huge; however if you are trying to determine the exact length of a multi-megabyte plaintext, the savings are more significant...