You can change the size of the char field in the XML-file :

<field name="my_field" style="font-size:13pt;"/>

The Temur's answer is for the number of characters.

field definitions are incremental. it means if you know what is the model this field belongs to, then you can inherit this model in your python code, add field with the same name and in field definition use the only attribute, the size. this way size will be changed, whereas all other properties will be preserved. say it's a Char field named "ex_field", then:

class my_model(models.Model):
    _inherit="parent.model"

    ex_field = fields.Char(size=256)

as noted above, ex_field is field you need to change size for.