The default print option in CheckIn is to a standard 8x11 invoice. To activate receipt printing in order to print to a Star TSP receipt printer you need to go to Admin> Workstation>
At the setting tab search for "receipt" as shows below and select Y for "Use Receipt Printer? and add the workstations printer ID in the field below.
Once this is setup, you will see a new button called Print Receipt at the top of the invoice screen and on the POS screen if you are using that.
Also make sure your Paper Size is set to 72mm * 2000MM