Based on https://www.zabbix.com/forum/showthread.php?t=35699
it looks like real values for history and trends of numeric values are ~90 bytes, but zabbix documentation mentions different numbers, especially for history.
https://www.zabbix.com/documentation/2.4/manual/installation/requirements#database_size

For text/log items I'd suggest 500 bytes size, just based on experience.
adding a note that it's not possible to predict.

For events I'd suggest 170 bytes.

I suggest to finally improve it after we many times discussed it (could not find such discussions reported).