Hello,

You may try this,

First take one boolean field and use that field in your onchange function.It will True if your condition followed accordingly. then add attrs in your field in xml file which make your field readonly.

For Ex.

#add one boolean field
'boolean_field':fields.boolean('Boolean Field'),

#update your onchange function
def onchange_product_id(self, cr, uid, ids, product_id=None, size_id=None, context={}):
    res = {'value':{'boolean_field':False}}
    tmpl_id = self.pool.get('product.product').browse(cr, uid, product_id).product_tmpl_id.id

    if tmpl_id:
        sql = """SELECT * FROM product_ring_size_default_rel
                   WHERE product_id = {}""".format(tmpl_id)

        cr.execute(sql)
        if not cr.fetchall():
            res = {'value':{'boolean_field':True}}
        return res


#add attrs in your xml file
<field name="boolean_field" />
<field name="size_id" attrs="{'readonly':[('boolean_field','=',True)]}" />

Hope it will help you,

Thanks