I can't emphasize the importance of this step enough. You can't have a giant board game without giant dice. The idea is just ludicrous. Get foam dice, fuzzy dice, make cardboard dice, but get them.
Most childrens' board games have a path to follow with various pitfalls and bonuses along the way. Make up your own, or try to recreate your favorite family game life size.
Almost anywhere. We like to get them from here because of the variety available. It's definitely more fun with a big selection of colors.
Try searching for Interlocking Foam Mats to find more buying options.
This is meant to be an energetic and imaginative but safe way for your kids to have fun. So go nuts, but don't go crazy. Have a grown up supervise the action so it doesn't turn into the X-Games in your rec room.
Placing the tiles on carpeted areas works best to keep the tiles from slipping.