Currently, you get 5 points per day, to simplify things. Each point weighs a certain amount in that 100% total. You get 1 point for visiting, 4 points for 4 posts. Every post after the 4th doesn't count towards your activity, but that doesn't mean you should stop posting.

Currently the activity rating is based off your past 2 weeks of daily activity. Normally it would be based off your past month, but it was recently changed for a little while.

So, if you post at least 4 times a day every day for the next 2 weeks, you'll have 100% activity and the larger avatar. But be warned, missing even a single post will keep you from getting to 100%.