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I believe you can do that by following this guide: https://www.jotform.com/help/264-Create-Sub-Products-Based-on-a-Product-Option. However, the screenshots of that guide is using the older version of the form builder, the options and steps are still somehow the same with the new payment wizard. I will update that guide after this. But for a quick tutorial, you can follow the steps below:
1. First, make sure that you have setup quantity option on your payment wizard. Here's a guide about adding options to your products: https://www.jotform.com/help/225-How-to-Add-Options-to-Payment-Forms. After adding a quantity option, click the "Add Option" button.
2. When adding a new option after the quantity, you need to enable the "Create Sub Products from Options". You have the option to use the presets or just list your option options in the table.
3. If you need to set specific prices for the t-shirt sizes, then you can enable the "Special Pricing" option, here's a guide about it: https://www.jotform.com/help/265-What-is-Special-Pricing
4. Here's how it looks like:
I hope that helps.