I use a Wacom Intuos 3, 9 x 12.  I also have a Wacom Intuos 3, 6 x 8.  Both are USB.

The only advantage of a larger screen is if you make flourishes of  wide strokes with a brush, but if that is what you want be aware that it is really big.  My 9 x 12 is huge because of the added space around the 9 x 12.  There is a guide on Wacom's site to help you decide what would be best for you.  I think it is a good guide.   Just make sure your vision of a 19" tablet is really what you need.  It will be a two handful object, rather like carrying a large TV tray only heavier.  The only factor is what you are going to use it for.  From the applications you mention, I honestly don't think you need anything larger than a 6 x 8.

There are new Wacoms that are like portable LCD's that you can draw directly on the screen, and they can be used in a dual monitor setup.  There are 3 sizes and are very expensive.  I drooled over one for a while, but decided it was more money than I wanted to spend, especially after making the whole house over with Macs.

If you don't already have Painter, you can purchase Painter X bundled with a Wacom Intuos 3, 9 x 12 at a reduced price.  That is how I got mine.  However, now I think my 6 x 8 was good enough.

I believe all three of the applications you mentioned can work with a Wacom tablet.  Wacoms website indicate what programs will work with it.  One they don't list that I use is ArtRage.   AR works fine with a Wacom tablet.

This question has been asked many, many times and the general consensus is that Wacom Intuos 3 is the most used and recommended, and most people use a 6 x 8.

Best wishes on your purchase when you decide what you want.