As always, it's best to design at the largest possible size then reduce accordlingly.

Personally, I think you'd be fine designing at the largest size, saving that largest size as a PDF and then reducing for the rest of the smaller sizes as needed - just so long as the smaller sizes are proportionate to the largest size. If proportions are off, meaning when reducing you end up with wider margins left/right or top/bottom, then you'd need to design specifically for the size.

Keep font sizes in mind, too - if you use a 14 point font size as a footer on the largest poster, reducing that down proportionately could end up leaving you with something like 6 point or smaller on the smallest size - obviously, that's not a very legible type size, so keep that in mind. You may find you need to at the very least keep your text separate from the graphical elements of your posters to ensure legibility when scaling down.

If you use InDesign...

There is a feature that may help you if the other sizes aren't proportional.

After you make your poster go to Layout > Layout Adjustment > Enable Layout Adjustment

The resize the document via the document setup under File.

There may be a little correction and retooling needed after this, but this feature could save you a lot of time if you made all your posters in the same document saved them, and then applied these changes to them all at the same time.

Booyaka!

I'm a dunce about A sizes, but if your posters are really large you can get away with 150dpi for your images (flattened tifs, hopefully) and still have them look good but not take up as much memory.

----
Powerpoint is not a design application

as mentioned above, it may not be as simple as resizing, since not all page sizes have the same proportions. but you'll want to plan for that.

and you'll need to understand that things work differently at larger sizes than they do at smaller sizes. the impact of font sizes, in particular. there may be more work involved in changing sizes than you think, if you do it right.