I confess that I do not remember how exactly the first-year Contego shields work and what their limitations are (I do not think this has been elaborated upon),

They were directional, not particularly large but large enough for one or two people to stand behind, could not be fired through, and had to be maintained with the wand.

The larger versions expand that up to a full sphere that can be fired through and only require concentration to maintain once cast.

It seemed that each hit would drain the shield caster's magic, so your shield strength depended directly upon the strength of your magic.