Although any aspect of lolita can be debated, the steadfast, make or break rule is the silhouette. If the skirt isn't full, it just isn't lolita. But how much fullness, or poof, does one need? To answer that I present 4 photographs:
This first photograph is a base photo. It is a simple JSK (Jumper skirt) without any support. All dresses will hang just about straight down without something to support them. Although the outfit is reasonably well coordinated with a matching blouse and some jewelry, it is not a complete outfit because the skirt has no shape to it.
This next photograph is the same JSK with a simple cotton petticoat underneath. The cotton petticoat does not give any poof, but it does create the appearance of fullness. This is a good option for when you want to wear lolita but the usual amount of poof would be impractical. Situations such as travel, standing room only concerts, and school (high school/college/graduate programs) lend themselves to a minimal poof petticoat.
This third photograph demonstrates a fairly typical single layer net petticoat. Although this one I made myself (tutorial coming soon) it is similar in construction to this one from Baby, The Stars Shine Bright. This is an every day amount of poof. It gives a decent amount of fullness but is not unmanageable. It will fit on train seats, roller coasters, in cars, and chairs without trouble, but is more than acceptable in terms of shape and fullness. Please, do note this petticoat, and the next one, are A-line petticoats. They create a shape that gets progressively fuller from the waist to the hem, as opposed to a bell shaped skirt. Both are widely accepted lolita skirt shapes.
This last photograph shows the same JSK with a two layer net petticoat underneath. This petticoat is again homemade, but similar to this petticoat from Baby. This is a more formal amount of poof. This much poof, although aesthetically pleasing, may be impractical for especially active days. It is more suited for days when many photographs will be taken, teas, and meet ups.
It is always worth noting that to some degree the amount of poof is a personal preference. Some consider this level of poof the bare minimum to really be lolita, while others require less. What is important is that one finds the amount of poof appropriate for one's self and for the activities of the day. It is also worth noting that if one is not wearing some sort of support under their skirt, because any support is impractical, then perhaps the choice of outfit in general needs to be reconsidered. Lolita require support, and if the day's activities prohibit the wearing of support, then the day's activities prohibit the wearing of lolita.