D6: set max size for external images to avoid breaking theme? (e.g. in bueditor etc)

so i'm using bueditor in this case - but in my site the open text area also allows for one to embed an image (e.g. video or picture from some external site and so on)

how might one set the maximum display size for an image from an external source url?

for example, here's the code used in a BUEditor button

js:

// function that inserts the user input from the form into the textarea. this might be defined outside as well.
myUserInput = function(form) {
  E.replaceSelection('User input is: '+ form.elements["user_input"].value);
  editor.dialog.close();//close the dialog when done.
}

var form = [
{name: 'src', title: 'Image URL'},
{name: 'width', title: 'Width x Height', suffix: ' x ', getnext: true, attributes: {size: 3}},
{name: 'height', attributes: {size: 3}},
{name: 'alt', title: 'Alternative text'},
{name: 'align', title: 'Image align', type: 'select', options: {'': '', left: 'Left', right: 'Right', center: 'Center'}}
];
eDefTagDialog('img', form, 'Insert/edit image', 'OK');

i'm basically looking at width and height - realize that it would be *nice* to apply imagecache presets (this has been done for bueditor for example on the 5x release but for use with imce which is of zero interest) - i'm looking for a more universal approach that could perhaps be applied at the theme level...

could this be controlled via the theme's own style.css - set to override any external image/embedded item such that it can not exceed X by X (e.g. break the theme)?