To add size and colour options for your product, just click on the "Add Variants" option while adding products and you will be able to add different sizes and colours to them.
If you own a clothing store, this feature is very important as buyers can order as per their size and colour preferences.
In Size, click on the "Add Size" button and then you can use sizes like "Large", "Small", etc..It will also give you an option to select different prices as per the size.
This feature can also be used for adding pricing as per variants in all business categories. For example:- To list different variants of Sugar, you can click on size and add size as 250 grams or 500 grams and list prices accordingly.
In Colours, you can choose the colour palette and add colours accordingly.