I just found the solution in this forum. thanks

Create a file for the following path: module_name/static/src/css/pos.cssunder the pos.css file - 

pos-product-line {

    font-size: 14px;

}

You can also use t-style tag in your template to add the css

Copy codet-name="OrderReceipt" t-inherit="point_of_sale.OrderReceipt" t-inherit-mode="extension" owl="1">

    t-style="{'font-size':'14px'}">

        expr="//div[@class='pos-product-line']" position="attributes">

            name="style" />

It's important to note that this will change the font size of all product lines in the receipt, if you want to target a specific product line you need to use a more specific CSS selector

Hope it helps