How/where to change the default size of inline images?
I would use the Image Resize Filter or Image Autosize Filter modules. These module add a filter to your input format that automatically resizes images added through the WYSIWYG editor. Image Autosize Filter allows you to set a maximum width, which seems like it would work for your case.

As a precautionary measure, you could add some CSS like this as well:

.node img {
    max-width: 450px !important;
    height: auto !important;
    border:0px;
    margin:0em;
}