The notifications should have a maximum width for a few reasons.

 * It is harder to scan a large area of the screen.
 * If the message is too long it takes a long time to read it and it may disappear before then.
 * The "start" position for reading will vary between notifications
 * Generally one reads the entire thing to determine if it should be acted upon and that takes too long with a long message.
 * We want the message to be consumed in little more than a glance if possible
 * Currently it can even be wider than the screen

On very small screens it may make sense to consume the entire width of the screen.  However, on large screens this makes very little sense.  So, we probably don't want to use a percentage of the width to constrain the size.  Probably a certain number of 'em' units would be better.

I guess we have something like the following:
[App icon] [Summary] [Message]
           [actions...]

I'm thinking that maybe actions as buttons should be on a separate line.  And they should be infrequently used.  Perhaps we should also limit the number of them.

The icon should already be a constrained width.  So this leaves the summary and message.  Some messages may have one or the other or both.  So we should probably not try to constrain them separately but rather as a single entity.  For the exact width I think we're going to have to experiment a bit but maybe something like:
 Use the full width of the screen up to a maximum of 140em.

If they exceed this they should be ellipsized.  More of the message can be displayed in the details view.