Can i add additional field to sales receipt for internal use to capture customers shirt or shoe sizes to aid future marketing activities?

Hey there, @hassumang8.

Yes, you can add another line item in your sales receipt to track your customer's shirt or shoe sizes. However, in order to do this, you'll have to create another product/service item and make sure to set its price rate to zero so it won't affect your total sales. 

Although, you can use the Message displayed on sales receipt field in your sales transaction. This way, you can add details to your sales receipt without creating another line item.

All you have to do is to open or create your sale transaction. Then, add your customer's shirt or shoe sizes in the Message displayed on sales receipt section. Once done, click Save and close.

Additionally, there are two ways on how to you can record your sales transactions in QuickBooks. This usually depends on when you receive your customer's payment. To know more, you can check out this article: When to use invoice or sales receipt in QuickBooks. 

You're more than welcome to visit this thread again if need further assistance in managing your sales transactions. Just add the details of your concern here and I'll help you out.