MySQL, RAM disk for tmpdir - how to figure out what size it should be?

We regularly run a few large SQL queries to generate reports. It seems that the bottleneck is the amount of time that is spent writing to the tmpdir, which is currently on disk.

We are upgrading to a larger server and I would like to setup tmpdir to use a RAM disk, but am not sure what a reasonable size would be to accommodate these reports.

Is there somewhere in MySQL that might log the most amount of space tmpdir it uses, or somewhere else I should be looking to calculate this.

Thanks in advance for your help.