Hi! thanks for your feedback. You are correct, that I do not give much explanation on what a Big Fig is, so I have made a couple minor edits to the text, as well as updated the URL to a properly hosted site. go ahead and download the latest version.
To answer your questions here, though, a Big Fig is a huge thing that is three times or more the size of a regular size minifig. Examples might be a dinosaur, car, AT-AT walker, huge rediculously big giant, rolling catapult, etc. A horse, troll, or motorcycle would NOT be a big fig.
Also, a standard minifig has 4 hit points. More durable things have blocks of 4 extra hit points, for a total of 4,8,12,16 etc These "blocks" of 4 hit points are killed one at a time. Make sense? Note that something doesn't have to be a Big Fig to have extra hit points. Batman might have 2 blocks of hit points just cause he's tough as nails.
What can a Big Fig do? Well whatever the players agree it can do. Legos are about creativity, so i don't want to bog things down with a bunch of rules about what you build and how it works. If someone puts a giant robotic spider with lasers for eyes on the battlefield, everyone could agree it has 3 blocks of hit points (for 12 hit points), can shoot two lasers (count as rifles) out of its eyes, has a "power" of +1 armor, and can move 12". If someone wants to fight a triceratops, you can say he can attack any one target with horns for 2 dice of damage, stomp another target for 1d of damage, has +2 armor, +1 defense, 4 blocks of hit points (16 total) and moves 6". Whatever you want!
I don't want to stifle the creativity of the players on how many hit point blocks your super awesome construction has, the main things is the players should agree what things are worth before the game starts. i'd appreciate any more feedback you have.