Hi Mate and Welcome to SD!
There's two options, use some existing bio media from your existing tank to seed the new filter, essentially giving you a cycled tank pretty much straight away. However because the colony would be small it would need to be fed and grow to take the bio load from the discus.
The other option is to start from scratch, this way you eliminate any chance of cross contamination from the bio media to the new tank. Personally I would forget any other fish species or aquatic life for that matter.
Size really depends on what you want. Personally I am impatient and I wanted big discus straight up, as they were going into a display tank and also didn't want to do multiple water changes daily. I would go for a minimum of 6 as recomended by others. Keeping in mind the 1 discus/10 Gal rule of thumb. Bigger is always better, and 75g seems to be a good starting point for most. I always say/think, discus are expensive, so why go cheap/small on tank and equipment?
I hope that helps.
EDIT: Also, work out the best and most effective way to do waterchanges regularly. If you're not going for an automated system, make things as easy as possible. Have dedicated drain and fill hoses from aging tank, big pumps to drain and fill tank quickly (but not too quick!). I can carry out a 300L waterchange in around 3-4mins.