Output size is currently limited to 4000 bytes at protocol level. Given the maximum transaction size Trezor can handle (~9kB), we also want to enforce this size limit here so that when the limit is raised at protocol level again, Trezor would still not be able to produce larger outputs than it could reliably spend. Once Cardano-transaction signing is refactored to be completely streamed and maximum supported transaction size is thus raised, this limit can be lifted.
