The product page offers two sizes for each product. For example Coca-Cola 1liter and Coca-Cola 0.5liter. Above is the productname "Coca-Cola" and beneath of that comes the "Coca-Cola" description. Is it possible to insert both sizes into the schema.org markup or do i have to choose one productsize and just ingnore the other size/price?
I assume that you refer to Thing > Product
Product is defined as:
anything that is made available for sale — for example, a pair of shoes, a concert ticket, or a car.
In e-commerce world anything that is made available for sale is called SKU (stock-keeping unit) and product is usually a container for SKUs (I said usually, as different platforms can have different terms or data structures).
So from your description I see that your system has a product which is "A bottle of Coca-Cola" with two SKUs:
I would use information of the SKUs for schema.org Product.
Some schema.org attributes comes from parent product (in your case A bottle of Coca-Cola) e.g. url (assuming you have two SKUs: 1-litter and 0.5-litter on the same page) or description (assuming you have generic description coming from product). But some other attributes like weight (or size / price) will be specific for each bottle. There is also sku attribute which usually refers to your SKU number.
As a side note, you can find similar challenge when building, say, Google Product Feed, and the similar approach will work perfectly fine.
I hope that will help.