Hi Colleen,
Thanks for the description. Unfortunately, changing the field length via Survey123 will require the republishing of the service (as the field needs to be re-created). An alternative would be to, in ArcGIS Online:
1) Add a field to temporarily store the existing values
2) Calculate the values from the existing field to the temporary field
3) Delete the existing field
4) Create a new field with the same name as the original field and with the new length
5) Calculate the values from the temporary field to new field