This will be 'Bottle Size' and will allow me to enter 75cl 90cl, 120cl or whatever size.
If I understand this and it is similar to other types of issues.
You have a bottle product that is available in several sizes, Now one can create separate products for each OR one product with optional size selection ability.

If this is the case then attributes created for size and added to these products from the admin so that the customer is provided and sees the options to select from is what you need.

To do this correctly the tutorial outlines this for size as an example. Just follow closely the steps