I don't think so, you can increase it without exposing as the property is part of BLOB object as per OOTB configuration. Though you can expose and modify the size of the property in the respective table.
I believe the property which is available in data table will be exposed by default, that is the reason why it is allowing you 255 characters only. So i would suggest you to cross check the property in your database to make sure and if it is already exposed then you can increase the column size to your requirement.
Are you confusing yourself with ASCII values(which contain 255 characters) with text input limit? I tried the same thing as you mentioned and I am able to save the property without exposing it. I am not sure if this is the information you were looking for, so please let me know in case of further queries regarding this.