tradestation 9.5
and what is this called?

jiaqiangmit,

this is the "Data Tip" window and Tradestation has not built-in option to change its font size. In Tradestation 10 you might be able to enlarge it by changing the windows scaling settings, but this will likely affect more than just the Data Tip:
https://community.tradestation.com/Discussions/Topic.aspx?Topic_ID=155888

Regards,

ABCTG