Make Notification More Prominent Since I have a fairly large screen, I often miss appointments, even though Calendar showed a notification. But the notification is very tiny on the large screen and I ofter overlook it while being concentrated on my work.

Do you know any tricks or can you recommend any tools to better make me aware of Calendar notifications?

My ideas so far include a flicker of the whole screen — like some Terminal softwares do when encountering a bell signal (\b). Or maybe a fullscreen notification. Just about anything would do that catches my attention even if I'm extremely focused on what I'm currently doing.

(Edit: It seems someone else had this problem before: Create More Aggressive/Persistent Alerts)
I was hoping that the Accessibility preference combined with Notifications sounds would induce a screen flash but it does not...

However, have you looked at using Growl (for notifications) and OmniGrowl (specifically to enable iCal notifications)? This is good if you mainly want iCal notifications (doesn't work with a lot of other apps) and especially if you'd like to have a more notification-based desktop including other apps and want a more granular level of control over how your notifications appear to you. Growl has a lot of nifty other things it can do that can help with and it's super customizable.