How do I allow users to select the size for separate parts of a product (e.g. for a suit)?

To allow users to select sizes separately for a product, specify all size combinations in the product options.  

Let's look at an example with a suit.

Create a product card in the Catalog. Catalog → Add product:

Add variants for the product. To do this, click on the Manage options button in the product card → Add a new option:  

Create the "Trousers Size" option using the same steps.

Next, list all size options for the suit parts using the Add a new product variant button, as shown in the screenshot below:

Save the changes in the product card. Now the user will be able to select the size for the jacket and trousers separately. The sizes will be displayed separately for the order in the shopping cart:      

The described option is available for products added via the Catalog feature.

https://help.tilda.cc/online-store/catalog