This is really just a repeat of an old feature request topic that has now been locked (and presumably rejected).
See here: viewtopic.php?f=12&t=28270
I can't overestimate just how useful this feature is if you have to crop a lot of images for the web. From reading the previous thread it seems like there is some confusion and misunderstanding about what exactly is being asked for. So I'll try to explain it again.
Just for reference, here's a screen grab from Photoshop CS3
This function is part of the crop tool. What it lets you do is define the exact dimensions of cropped image before making a selection. It's important to note here that the pixel size of the selection box which you draw is not constrained, however the apsect ratio is, so that the cropped image always conforms to your specified dimensions.
This is very different from a fixed size selection box. A fixed size selection gives you no flexibility over the portion of the image contained in the cropped version because there is a 1 to 1 mapping between the pixel size of your selection box and the pixel density of the image. Imagine you have a photo of a group of people and you just want to crop out a face, and also, you want to crop the photo to be exactly 300px by 220px for a website. As things currently stand, this is how you would achieve this function in Paint.net:
1. Use the selection tool in normal mode to select the part of the image you want
2. Crop to selection
3. Resize to the desired width (because all photos on your website have to be 300px wide)
4. Use the fixed size selection using a width of 300px and a height of 220px to select the vertical portion of the image you want to keep
5. Crop to selection
There's also an alternate way to do this:
1. Define a fixed ratio of 300x220
2. Crop to selection
3. Resize "by absolute size" and set the pixel width to 300px. The height should automatically adjust to 220px (although i have a hunch you might sometimes get a rounding error here and it could be out by a few pixels - not absolutely sure about this).
You now have the desired portion of the image cropped to your required size. Using the crop to size function in Photoshop all of these steps are condensed in to 1 step. If you have a lot photos to crop this function is a massive time saver (and it gives you more creative control with less trial and error involved).
I really like Paint.net, but I'm a web designer for a magazine who's been using Photoshop for over 10 years this is the feature that I really, really miss. If you have to quickly crop 50 images it's a lifesaver. Please, please implement this.
Thanks
(Gimp can't do it either btw!)