It is not possible to change the field size of any of our out of the box fields.
As Carol mentioned, create a new extended field with a different name from the OOTB field named Purchase Order, and give it a longer field length.
Next, create a data import file (csv) containing columns like asset model name, asset serial number, original purchase order id field, new purchase order id field, and any other fields needed for an asset import.
To create the data import file, create a new asset search that finds assets where the original purchase order id field 'has any value'.
Add the fields needed to this search, and then export this to a CSV file.
Then create a data import that updates the NEW purchase order ID field with the OLD purchase order id field data - and use the CSV as the data file.
Lastly, change the asset configuration to HIDE the original purchase order ID field, and show ONLY the new purchase order ID field.
Hope this helps,