a) How do i increase the post area size? (That is the written matter area size shud be more than given to me...want to reduce the overall sidemargin of pages a bit)
This is located in your theme's "style.css". Either download it or go into the theme editor in admin (the theme files must be CHMOD 666 for this option). Look in styles.css for something like this (may not be exactly):
margin: 0 auto;
padding: 20px 0 10px;
/* The entire template's width is set in this class. */
border: 1px solid #ddd;
Play around with the width size a little bit at a time.
b) The sidebar thingies are coming very miniature in size. Dunno if its a problem with my browser or not. Actually, once i posted a post with 'georgia' font and the sidebar automatically looked good. I dont know what the connxn betn the two is, shudnt have happened. But it did.
Would need a link to know what you're talking about.
c) Just in case, i want to change the image on top of the page, how do i do it in this theme?
You would need to find/create an image exactly the same size as your current one. Name it to kubrickheader.jpg (this is what your current theme's header image is named) so you don't have to do any coding. Rename your current one if you want to save it. Upload your new "kubrickheader.jpg" into your theme's images folder.