Well ... the upcoming 3.30 release (currently in beta - see this news release
for links to the appropriate edition) allows you to set the don't fragment flag - and then you can manually change the packet size yourself to find out the maximum.
A little "hidden tidbit" here. If you hold down the control key when you launch the options dialog (Edit -> Options...), then the options dialog stays on top, but it still allows you to interact with the main user interface. You can then change the packet size, and hit "Apply" to see the affects - you can still do things with the main user interface then, even with the option dialog visible. This lets you easily try a few packet sizes until you've found the right MTU.
Hopefully, this does some of what you're after.