Post Page not displaying correctly

Recently updated to the new version of onetone and now my post page has completely changed.

Instead of showing a small picture for example, the featured image is 600×450.

Also, all of the posts are fully displayed instead of automatically having excerpts.. I tried adding a function to the themes functions.php to force a 75 character excerpt however it didn’t change anything..

What I’m looking to do is:

Force all posts to automatically have an excerpt of 75 characters
Limit Featured Image size to around 450×300
Add Sidebar to postspage to display categories and subscribe option

Any help would be greatly appreciate.

Cheers