iPhone e-mail photo resizing — which one should I pick?

So the iPhone running iOS4 now offers to resize any photos attached to an email, just after you hit the send button. Quite a useful feature (does anyone really need a full 3 or 5 MP picture of your latest meal/injury/novelty vegetable/...?).

It offers

• Small (~10's of kBs),
• Medium (~100's of kBs),
• Large (~1000 kB), or
• Original Size (2, 3 or 5 MP depending on your iPhone model)

But it doesn't actually tell you the dimensions of the images, only file sizes.

So for those interested, here are the dimensions in pixels, and approximate viewing sizes at three different resolutions: 100, 163 and 300 dpi, typical for a monitor, iPhone and a good quality photo print respectively.

Small — 320 x 240 (8 x 6 cm, 5 x 3.7 cm, 2.7 x 2 cm)

Medium — 640 x 480 (16 x 12 cm, 10 x 7.5 cm, 5.4 x 4 cm)

Large — 1280 x 960 (32 x 24 cm, 20 x 15 cm, 10.8 x 8.1 cm)

(all figures rounded to 1-2 sig figs as I felt like at the time, the dpi's are similarly accurate by the way — do the maths yourself if you need accuracy!)

It may be worth noting that the iPhone 2G/3G/3Gs has a screen resolution of 480 x 320, so even the small one will fill a decent chunk of that screen, and medium is ample to cover it. The iPhone 4 has a screen resolution of 960 x 640, so medium will fill ~2/3 of the screen and large will cover it entirely.

So how does it look at full screen on your phone? (i.e. how you're seeing it by default in the Photos app)

• If it looks okay and you have no desire to zoom in, small or perhaps medium sized images will be fine for anyone.
• If you need to zoom in to see about 1/4 of the image at a time, send a large version.
• If you're zooming further or want to print it, send the original size image.

While their device might have a different resolution, if all the detail is there in your full screen image, then it'll be there on theirs.