Oh, I looked at the specs. The 16 x 24 doesn't have an upper floatational platen or central pressure adjustment, where as the 16 x 20 does. The upper floatational platen means that it is easier to print on thicker garments. The upper platen will wiggle, so don't think that your press is broken if you order it. Also, central pressure adjustment is much more accurate than the 16 x 24's high pressure adjustment. For the 16 x 20, the pressure adjustment knob is in the middle of the upper platen, so when you change the pressure, it changes it evenly throughout the entire platen. A high pressure adjustment knob won't do that as accurately. So, I wouldn't risk having any problems like not being able to print on something thicker, or some parts of your transfer being more faded or less opaque than other parts. Buy the 16 x 20.
^^^ was mostly for jrtees1's question, but illsauce, I hope this helped you too.
And, illsauce, printing your own designs from your inkjet printer is much more common than ordering custom plastisol transfers online. JPSS is just a kind of transfer paper for lights, the one that people have had the best results with. It looks great, the colors are vibrant, and it's definitely much cheaper than plastisol transfers. You can get 100 sheets of JPSS for around 60$ (shipping included.) I order from alltransfers.com, they're the cheapest place I've seen. For darks you would have to use Jet Opaque 3g. Trim your JPSS 1/8 of an inch, and trim your Jet Opaque perfectly. This is all the important info I can give you. You'll be able to find a lot more info about inkjet heat transfers on the site. Basically all it is, is that you get transfer paper, print out your design, and transfer it with a heat press. Make sure you get a good printer though.