Do you plan to use it from relatively static locations like a desk at work, at a coffee shop, - desk or coffee table at home without feeling like you need to hook it to a monitor? 15”.

Do you plan to use it on the go, primarily while traveling or otherwise want to be able to take it with you somewhere on a whim, or otherwise use it in more dynamic spaces in your house like from different sides of a couch, in your kitchen, sitting in the porch etc. then 13”.

To boil it down, 15” is great for when you want to use it as a more mobile desktop computer. 13” is great for “I need to take this with me everywhere, or be able to use it anywhere without feeling like I’m ‘lugging it out’”