The TR1 is best for small things that are thin at the top, like 6714 20mm stars.

TR2 is great for pieces that are not too thick at the top like 3855 20mm faceted balls

TR5 is perfect for Asfour's 3855 30mm,  3856 63mm, etc. 

TR6 is what you want to use for much larger pieces like  3855 50mm and larger.

The above are just suggestions, what really matters is how wide the top of the crystal is. The best solution is to buy a few of each if you are not familiar with the crystals you are purchasing.