Broadly the principle is that you need a new GTIN if anything changes in branding or the functional description of the product or in anything that you declare to the consumer such as weight.
If any physical dimension changes by 20% or more you must issue a new GTIN but since size changes can have significant logistical or display implications you should consider changing the GTIN in any instance of size change. At least you should advise your trading partners of the size change and enquire whether their preference is for a new GTIN.
There are many nuances however and you are advised to consult the full document..
Check the GTIN allocation rules in the GS1 global website.
Version 1.0 – Updated 25th October 2015