From the document https://kite.trade/docs/connect/v3/orders/#placing-orders, the tag associated to each order is mentioned as 8 chars max,. but I did try more than that to 20 chars and it worked. For sample, I tried to hit it with the KITE sdk with below details
order id : 200128002847996 sample random string tag value: 111sedrftgyhujikolpq