I hear you on blogging sites. I used Blogger for awhile and WordPress is like night and day.
In wordpress for your first question, go to settings in the dashboard, and then select Media. Thumbnail size is first option.
The second one is a little tricky as you are using a pretty customized looking theme. Theme Options also under Settings, might allow you to customize different features of this theme including what opens up when you click Popular and Comments. Maybe you also have to enable the widgets for comments?
