I've done a recording of a 1Gb raw traffic file before, when I had a retail web app that cached the whole database on login. DevTest is written in Java, so I had to dramatically increase the JVM maximum memory to permit this. You will see if you need to do this because the generation will start at normal speed, and then slow to a crawl when the JVM runs out of memory. You will also get errors in the log file.
The file to increase RAM is DEVTEST_HOME/bin/Workstation.vmoptions. I had to add the line "-Xmx3g" (without quotes) to this, to provide DevTest with access to 3GB of RAM.
The raw traffic file is just XML, so if you have an editor that can cope with files of this size, you can split it, so long as you remember to include all the XML prefix and suffix tags in each of the raw traffic files you generate.