When I give presentations, I have to keep an eye on the clock to make sure my speech does not take more than alloted time, or in the end to know whether I can take more questions.
Looking at my watch is inconvenient, I would prefer my Android phone's large screen to:
Sounds trivial. Is there such an app?
Priority to free apps, biggest priority to open source apps.
Digital Clock does almost everything you ask. Ignore the rubbish picture from the page, it is a full-screen clock with white text on black and it's free.
To configure it as you need, launch the app, then open the settings (tap the screen to open the options then tap Settings) then set:
You can also adjust the time format and colour if you wish. The time is about half the screen height, but I can easily read it on my phone from more than 20ft away so that may not be a problem.
I use Ovo timer: a minimalist app to set your timer by dragging an arc on your screen. Also, you can set time by voice (it needs Internet connection for this). It runs on Android 2.1 and up. The only con I've found is that it only supports times not greater than 1 hour.
About what you're asking for:
I use Stopwatch & Timer Goes full screen when you change into landscape orientation, also I used the lap times when we had to give a team presentation.
I use Flow Timer for this, it displays the (countdown) time in the largest possible width in black on white (default). Furthermore by default it just dims the screen but doesn't turn it off (in the settings there is a choice to keep it at "bright", that is, to not even dim the screen a bit). Landscape mode (even tablet) as well.
You can also add several timers after each other for different parts of the presentation. No interaction required, it just makes a small beep (configurable) and continues/finishes.
Full disclosure: I'm the author