From what i know there is no way to change size after criteria.
But what you can do is to make it double row of that field in the portal. cant tell for sure what layout you have.
is there a way for the field to automatically extend to display the full text in it??
There is no such setting.
A partial workaround is to use Conditional Formatting to use a smaller font size if a certain length is exceeded.
This is where a Tooltip may help. (a temporary view of the contents upon hovering the mouse over the field)
Have a button show a popover to display the full text.
In addition to tooltips and popovers, you have the option to give the field scroll bars or you can simply click into the field to make it pop out large enough to display the entire contents.
The main problem with fitting text to a portal field comes when you want to print a layout with a portal. If printing is the issue, more complex solutions are needed. Sometimes you can redesign the layout to use a list view based on the portal's table, other times, more extreme measures are needed.