"Large" files are rarely anything but illegal. There are few legal files that are large. In any case, the fact that 99% of downloads done in mIRC *are* illegal means that we don't help anyone with DCCs other than very basic information per that sticky. That's what other forums and channels are for.
Anyhow, if your download is timing out, there is little you can really do about it. It will probably be your ISP that's causing loss of connections. The best you could do is switch to another download method that does not need a semi-constant download stream to keep downloading, or else find a better ISP. Note that, as mentioned, this could also be the sender's ISP problem.