Glad I found this post- looking to do the exact same thing by passing a large text field through an activity call (aka the internal event form submission calls). Looks like you can map a large text field when you create a regular external call but you cannot map it when you're leveraging an activity call from an internal event. The purpose of using the large text field is when people submit a contact us form, they leave sometimes lengthy descriptions and the small text field doesn't leave quite enough room for them to provide us that information. We don't want to cut any of the data off by transferring a large text field to a small text field for that reason. Plus it seems to contribute to a messy environment.
Did you find a workaround for this, Neelima Manthena? I'm not sure mapping a large text field back to a small text field is the most optimal way, so I am curious if you found an alternative method.