Hi,
The default value of a custom field can be changed by editing the custom field in the menu Display>Custom fields. There, you have a setting "default" where you can enter/select the default value of the custom field.
There is no limit setting for the length of the text you can enter in a custom field.
However, it is limited by the type of the column corresponding to the custom field in the database.
By default the type of the column is LONGTEXT which gives a maximum length of 4,294,967,295 characters.
So if that's what you're talking about, I would recommend checking the column type for that custom field in the database. If it's set to "VARCHAR" then the maximum number of characters will be 255 characters.