I have a 200 litre tank, and a 200 litre jewel filter - but 6 young (less than 4 inch body length) fantails.  I know that the tank is too small and will get a larger tank when i am able, but would a larger filter, or additional filter help in the short term, what size should i get?

I think Bubba didn't mean twice the size physically. LOL
But if you do get the filter system that is to handle twice the size of the tank make sure the current of the water coming into the tank from your filter is not going to put your fish in a washing machine motion.

It's really tough, changing the water too often can cause stress too. In any case I think you will end up wasting money to work around it. So I would say go get a good size tank for the fish instead so they and you can relax and enjoy. Some tanks can be cheap when you buy used ones or in some case a store has a sale.

I was lucky a store near me was moving and they sold a bunch of big tanks for cheap so they don't have to move them. Again, I was lucky. Maybe you can find someone who is upgrading their fish to a larger tank and can sell you their previous tank for cheap.

Good luck.