The only major advantage of using the bigger dies is that if you have a set that includes some larger sizes, you still only need one die stock.
In practice this is soon negated by the extra cost of the larger dies!
The larger dies are a bit thicker, and this can be an advantage when working with coarse threads such as BSW or UNC, but in practice the smaller dies are fine for most applications, and can even be an advantage!
Most dies seem to come in 13/16, Inch, 1 & 5/16 and 1 & 1/2. These sizes will cover most of what you will find around the average workshop. Worth acquiring die stocks in all these sizes, but don't bother with new! These things don't really wear out and all my larger ones were acquired second hand at low cost. Most of them are better quality than the new ones anyway! (Don't be afraid of a little surface rust, but avoid anything heavily corroded or looking like it was used by a Sussex farmer for threading badger turds!)
Adjustable dies are no problem. When you put them in the stock there are 3 screws. Turn the centre one (which also stops the die rotating) and it opens the die. Slacken it, then wind up the other two, and they close the die!
Generally, you don't need to adjust the die. Just plonk it in, and turn the middle screw so that it's holding the die, but not spreading it. Most times you can use it like that, but it's best to then adjust the other screws till they are also holding the die too.
95% of the time, that will do you! About 4% of the time you will need to make the die smaller (the nut feels tight) and about 1%, make it bigger (nut feels loose) but this is rare!
