Malleable or not is a function property that doesn't make sense for OTPs. The problem is that while a OTP's outputs are determined by its inputs, a OTP's input

Malleable or not is a function property that doesn't make sense for OTPs. The problem is that while a OTP's outputs are determined by its inputs, a OTP's inputs are always different. That said, a one time pads are non-malleable if you don't reuse the bits (and the bits are truly random). If you do reuse the bits, it's not a one time pad. That said, there are no length issues with a One Time Pad. Or rather, the length issue with one-time-pads is that you can't reuse pad bits, not just within a given message but across all messages that ever use that pad, hence the name ONE TIME Pad. One time pads are necessarily as large as the data to be encrypted. (You might be able to get away with some fraction of the total number of "to encrypt" bits, but you can't reuse them.) Schemes which reuse "encryption bits" do not have that limit and are much smaller.