It’s agonizing to try to come up with a joke about what dimensions your slides should be. Just agonizing.
Your smartphone in your pocket has greater processing power than the computer that sent humans to the moon in 1969, but every single projector you will ever touch is basically just puking twelve pixels up on the wall and calling it a day. Projectors suck.
The vast majority of conferences and meeting rooms you will present in will have really low-fidelity projectors. Don’t bank on projecting on a fancy, bright, widescreen projector; you’re more than likely going to be disappointed.
4:3 or widescreen?
Should you present in widescreen or the more traditional 4:3 aspect ratio?
I’ve switched to writing all new slides as widescreen. It makes your slides look more cinematic, which I would argue feels more polished.
The major concern here is how those slides will work on an older projector. Keynote in particular does a great job at scaling your slides. I haven’t experienced a problem on any projector since I started this; 4:3 aspect ratio projectors simply show my slides with black bars on the top and bottom, which is reasonable and preserves the cinematic feel.
I tend to opt for bigger slides: 1920x1080. That gives me full 1080p resolution, so if I ever do end up presenting on a nice, modern projector, my slides will really pop. You can always scale down high-resolution slides, but it’s much harder to scale them up.