What's the range on each of the age categories? Are they always used?

First, I assume you are talking about adult comps in the U.S. There are 5 age classes:

Class I is 21-28.
Class II is 29-35
Class III is 36-45.
Class IV is 46-55.
Class V is 56+.

Depending on number of entries, skaters may be separated into age groups or combined into one group (usually that happens in small club competitions where there aren't that many adult skaters). Sometimes, 2 or 3 age categories can be combined - for example, at NYI next weekend, Bronze Ladies are combined into a I/II group and a III/IV/V group. In Pre-Bronze Ladies, there is a II/III group (no I's entered) and a IV/V group. At Adult Nationals, every age category is separate, due to the large number of entrants. However, I did notice that there was a note on this year's announcement that said age classes could be combined at the referee's discretion, which is a standard rule at all comps. Usually, that happens with the men's groups, simply b/c there are fewer adult male skaters than adult female skaters, and it's harder to get a critical mass (at least 2, preferably at least 3) in each age class at each competitive level.

These categories only apply to open singles events, though - not Champ Gold or Champ Masters, or Pairs and Dance events. Age classes there are always combined.

Which group generally has the most entrants?

Since this is the first year with the new age groupings, it's hard to say which is the largest. However, if I go off of the size of the groups at DBNAAI last weekend, I'd say III is the largest.

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I've got mad salchow disease!

the former 2 (and now 3) age categories have traditionally been the largest

I concur with flying~camel - class III is usually the largest. It will be interesting to see the effect of subdividing the old class I (25-35) on the size of the groups at AN this year. At both HC and NYI (and I think Peach, too) I and II were combined in Bronze and Silver Ladies (previously, those levels split I and II at those comps), so it will be interesting to see how the numbers between the 2 classes break down.