Featured Image Size Isn't Changing?

Hi guys,

OK – So I’m officially losing the will to live! 😛 Most annoying is the fact I know I’m probably missing something really obvious.

I’ve built a WordPress theme using one of the default framework themes that come with WordPress and it’s looking just about finished – except one thing.

On the main blog page, the featured image isn’t formatted properly – it’s just sat at the top left of the box and isn’t much bigger than perhaps 75×75?

Also, when you click through to the posts the featured images are still 75×75 at the top left of the page – no larger?

It’s frustrating as the theme looks really nice, it’s just this makes it look really silly.

I’ve tried applying some changes to my functions.php file and I’ve regenerated all thumbnails. I’ve also taken a look at some of the codex but am slightly overwhelmed by it all and have gone back to stage one.

Does anyone have any idea what code I can add / edit (and where it is) to set a default size for post thumbnails and featured images please?

Please don’t think I’ve been completely lazy on this one – I’ve searched Google and tried a few things, however most of it has been written about older versions of WordPress.

I’d be immensely grateful for your help guys, thanks a million!