Reducing size of thumbnail

Hi,   First of all, great work on the theme as well as the forum here! Really helpful :)   About the query, i want to reduce the thumbnail of the images shown on my blog page, so that i can accommodate more images/posts on one page. Will be even better if there is a way to showcase multiple category posts divided from one another (like a magazine theme). Here is the link, look forward to your support - http://www.smilespreaders.org/