Thank you Jacob. I dont really use small thumbnail sizes and I already upload at 1200px and wordpress already still makes a 768px size even with all defaults set to 0 so perhaps I just need to let wordpress create a medium size for mobile screens then, but what is the best size to use – would that be what the default medium setting was (300px) ?
I said earlier that I thought my theme was creating a 1024px but just realised that is the default wordpress large setting too.