Currently, email description field doesn't have the option to edit its Row Layout, and there is no supported way to expand it.
There's a way to edit customizations.xml as introduced in the page below; however, it is not supported and it doesn't work if the description field is set to read-only.
https://community.dynamics.com/crm/b/crmpowerobjects/archive/2014/04/15/how-to-expand-the-email-description-field-in-dynamics-crm

It'd be nice to have the Row Layout option for Email description field since the default height is not big enough for the email body.