When you use the "other" option, it treats the question as a relevant question, although not sure where to look for the extra field and its properties. You can see them in survey123 Connect if you go to the Schema tab of your survey, but you cannot modify it there. There doesn't seem to be any entry for it in the xlsx file though. Until someone tells us where to modify the properties for the hidden "other" field in the xlsx file, you have 2 options:
1. Modify the schema of the created feature service in AGOL. You can follow the steps detailed in this blog post. You will need to know the name of the "other" field to modify its values, so make a note of it first. Once you updated the schema, you need to download the survey again to the device and it should honour the changes.
2. Instead of using the "other" option, simply create a relevant option for the question during design mode to define the field you want to display when "other" is selected. With that you can define its field length and other parameters in the xlsx form.
I hope this helps.
Zoltan