Does knowledge of original file size provide a cryptographic weakness?
Yes, feasibility to guess the plain text size might be a serious vulnerability in real life scenarios. For instance, in traffic analysis the approximate length of the messages in a communication, might reveal enough information about what is communicated, for it to be possible to deduce the gist of it. If such threats exist in your case, however, you will probably have to consider more radical counter measures than just apply a few byes of PKCS#7 padding at the end of your file fragments.