Controlling Image Size for Posts in WordPress

I’ve been playing around with WordPress. As a programmer, the media library seems a little strange to me – but I’m keen to understand how I can achieve what I need with the framework. Cue WordPress enthusiasts…

I need to force my WordPress post images to be a certain size. I’m not talking about the thumbnail size, which I’ve found can be easily manipulated within the loop, but the images that actually get inserted into a post, amongst the writing and the other media. I have a fixed content width, and I would like the images to reflect that width (uncropped).

Obviously, I could run some sort of Regular Expression that could identify the image tags, and replace them with new sizes, but is there really no other way of forcing image sizes within the post while retaining there position amongst the formatted writing that comes with it?

So far, I’m guessing that there is a CSS property out there that is used by WordPress in all images. At a push, I’m willing to go with this – whatever it may be (answer needed). But I was hoping that their might be some more involved code (perhaps something I could plop into the functions.php file) which could actually force any inserted post images to be a certain width.

Any thoughts on this would be much appreciated.