"Big, old, traditional flat" is the best. Adjective word order=quantity, opinion, size, age, color, shape, origin, material and purpose

Big, old, traditional flat would correct when used in a sentence e.g. He bought a big, old, traditional flat. Most native speakers use this order naturally without thinking about it. So if the order is incorrect it might sound a little strange, but will still be understood. https://dictionary.cambridge.org/grammar/british-grammar/adjectives-order

Hello, I think big old traditional flat is corroct

With the sentence you wrote. It looks like you are describing a flat(apartment) as traditional, old and big. Correct? If so they all are descriptors(adjectives). it doesn’t matter which order you out them in.

So according to adjective order , it's traditional big old flat