If you are able to get another if it runs out then a 3.9kg will be fine.
I think most people use the 6kg size - I carry two.
If you're going away for a week with hook-up then you won't use much gas - only for cooking.
If you don't have hook-up you will use lots of gas - cooking, hot water and fridge.
If you don't have hook-up you will use more than 3.9kg in a week.
We went away for a week last summer and used about 6kg.
Hope this helps.