Certain ranks/levels gives you certain avatar sizes.
If you donate $25 to MPGH, your avatar size increases.
The ranges would be from 200 x 250.
Which would be way bigger than Member Level 1.
In addition, as you post, you gain more benefits.
Being active can be a good thing. Helping people is another. Spamming, is not.
When you reach Member Level 2 (2500 posts), your avatar size increases.
When you reach Member Level 3 (5000 posts), your avatar size increases.
When you reach Member Level 4 (10000 posts), your avatar size increases.
The quickest way to get your avatar size to increase is to donate
This should be in the help section, I believe @Flengo