how can I use wget to download large files?

You can continue failed downloads using wget. (Provided where you're downloading from supports it)

Quote:

Say we're downloading a big file:

$ wget bigfile

And bang - our connection goes dead (you can simulate this by quitting with Ctrl-C if you like). Once we're back up and running and making sure you're in the same directory you were during the original download:

$ wget -c bigfile

Provided where you're downloading from supports it, you should get going from exactly where you left off.

Normally, wget has no file size limit.

If wget is failing, it is probably because you are running out of space on the pendrive. Do you have a free GB for the iso?

How did wget stop? Was there an error message? Did you perhaps allow the computer to go into suspend? If so, use wget -c as suggested by @dtmland.

Finally, wget does have an option to limit file size but it is not set by default. One possibility is that your sysadmin has set a limit by making wget an alias to something like wget --max-filesize N. To check if wget is an alias run alias wget. If it is, you can use \wget to bypass the alias and run wget directly.