How do I set up the sizes in the CSS for import?

I suppose that you mean CSV, not CSS.
There is no size option for products.
There is only product_lenght, product_width and product_height that you can set for dimensions.

You can.
I didn't understand that you were talking about characteristics.
Please follow the documentation about importing products with characteristics/variants on that page of the documentation:
www.hikashop.com/en/support/documentatio...hikashop-import.html

Could you provide the first few lines of your CSV ?

Could you provide the first few lines of your CSV and not only the first line ?

That's not the proper way of doing it.
You should have something like that:
product_code,product_parent_id, product_weight, product_keywords, product_meta_description, price_value, size, images, categories, related, price_curency_id
armani_underklader_herr;; Armani Underkläder Herr; 334549; 1; Armani Underkläder Herr; Armani Underkläder Herr; 85; m; 334549.jpg; 12; 150;
variant1;armani_underklader_herr;;;;;m;;;;
variant2;armani_underklader_herr;;;;;l;;;;
variant3;armani_underklader_herr;;;;;xl;;;;
variant4;armani_underklader_herr;;;;;xxl;;;;
variant5;armani_underklader_herr;;;;;xxxl;;;;

yes.

For one product you need several lines:
1 for the main product information.
1 line for each variants of that product.
So, if you have 5 sizes like in your case, you should have 6 lines (plus the header of course), like in the example I gave above.
Each variant line should have a different size and the main product line will have the default value of the dropdown in the size column.

How did you set the product_code to show int he headline ?

The variants information is displayed because you have the option "Append characteristics value to product name " of the display tab of the config activated.

In your CSV, you didn't set any product_name column so hikashop automatically creates it based on the product_code.
You should add a column product_name and have it empty for your variants and filled for your main products.