I have build an app for an organic buying coop and it works really well, but I am looking to add products that have size variations such as 200g, 1kg, 25kg with different price points. I would really like the user to be able to select the size from a choice option and then the price changes based on the selection.
I have searched this group and google but haven’t been able to find a solution, so wondering if this can bed done? 
Certainly possible. As long as you have a unit rate, you’d just use a math column to multiply the unit by the rate and display the answer to the end user.
Thank you Robert for your answer but unfortunately the unit rate is different based on the sizes as it gets cheaper the more you buy. For example
Plain Flour
List item 1kg $4.95
List item 5kg $22.25
List item 25kg $77.50
So I am not sure if I can use columns on the product line on the google sheet to list the prices and sizes separately and this can then be used to offer a choice? Currently I just use rows and the items are listed separately on the app with the different sizes. I am just wondering whether I can have an option to select size and then the price shows?
Yes, have one table for items (with an item id) and another table for item variations (with item ids and prices). Create a multiple relation linking the two tables. On the form screen, point the choice component to the relation.