RE: Field Points
You either have to know or measure the diameter of the arrow you want to shoot. Then pick the appropriate size point. If you're thinking about carbon arrows then most measure around .295" which equates to a point diameter of 19/64". The two most common sizes used on them are 9/32" (a hari small) or 5/16" (a hair big) because that is what is most commonly sold. It's a pretty rare shop that carries 19/64" field tips.
The smaller diameter ones usually use a 17/64" point. For this you can use 1/4" or 9/32". Again, neither of these is an exact match. To get the right diameter here you should use Easton RPS points for ACC 3-28 shafts.
Then there are the larger diameter shafts like Gold Tip 22 series. which by definition are 22/64" diameter. Use the appropriate 11/32" point.
For aluminum arrows it's pretty stright forward. 2213 for example is 22/64 diameter or 11/32. This is the largest diameter field point made so shafts larger than this have inserts that taper to accept the 11/32" point.