Indeed, consistent use of one type for lengths avoids issues because you need no casts. For in-memory data, the type to use is size_t since that is what e.g. memcpy(), strncmp() and read() accept; for a position in a file or a file length, off_t is a better choice, since that is what pread(), lseek() and stat() use. I'm a fan of unsigned data types in general, but using off_t for file lengths is the way to go. (It's not like you need defined overflow semantics for file positions, anyway.)

(You don't need off64_t if you compile with the proper #defines, which you should do on Linux.)