On the work order detail page (New Work Order —OR— editing an existing work order), the Description field has an expanding option in the lower right corner beside the character count:

Click this to expand the description to a popup window:

Click the expand option in the upper right to close the expanded window and return to the work order.
You can see the expandable description feature on these:
- Editing work orders
- New work orders
- New work orders for the customer portal
- Creating work orders from proposals
- Recurring work orders
- Ready to bill orders