Question regarding mobile game graphics

[Omega_Jet]

I have this question which is probably stupid but has me stumped.
Say I want to develop graphics for an IOS game. The resolution of the iphone 5s is exactly 640x1136 pixels.
My question is that should each screen of sprites put together have these dimensions exactly for the best possible graphical output? Can the screens be smaller and be resized when the game runs on the phone? Or is there some sort of scaling that is used?

I am sorry if this is such a really stupid question, but when I opened up a new image on photoshop with 640x1136 px dimensions to make a mockup screen, it really was much bigger than I expected.

[Seiseki]

Go for half or a fourth of the resolution and then scale it up in the game engine. Depending on how large you want the pixels to be.

[Ai]

I have this question which is probably stupid but has me stumped.
Say I want to develop graphics for an IOS game. The resolution of the iphone 5s is exactly 640x1136 pixels.
My question is that should each screen of sprites put together have these dimensions exactly for the best possible graphical output? Can the screens be smaller and be resized when the game runs on the phone? Or is there some sort of scaling that is used?

I am sorry if this is such a really stupid question, but when I opened up a new image on photoshop with 640x1136 px dimensions to make a mockup screen, it really was much bigger than I expected.

The key bit of knowledge you seem to be missing is related to PPI : that is, one pixel on your computer display is not necessarily the same physical size as one pixel on your phone display. Indeed, phone displays tend to have higher PPI -> smaller pixels, than computer displays.

Photoshop, GIMP and other photo manipulation software allow you to set the PPI of an image, so that '100%' zoom will correspond to 100% at the intended PPI, rather than '1 image pixel = 1 pixel on your monitor'.

[Seiseki]

Using PPI isn't guaranteed to give you a even pixel ratio though? You might be getting 4½ pixels per inch..
I just can't imagine this would be a good idea if you want to use pixel art.

And isn't PPI in photoshop just used for print? 

[rikfuzz]

Retina displays have 'double pixel density'.  (The actual PPI varies between screens anyway, so this is a nice simpler way of differentiating between retina style displays and normal ones).  For web stuff anything measured in 'pixels' is sometimes referred to 'css pixels', as it doesn't necessarily correlate to screen's pixels.  A button 100px wide will be drawn with 100 pixels on a normal monitor, but 200 pixels on a retina/DD display.

So you can just half the phone's resolution and scale up at runtime if you want to retain a pixel look, or just embrace the HD size, and have extra detailed sprites twice the size you were expecting them to be. 

Photoshop, as far as I can tell, doesn't do anything with PPI info outside of print, except maybe how it calculates font sizes. 

[Omega_Jet]

Thanks guys for all of the valuable information. So I guess going for half the resolution is probably the best idea. I will also look into PPI to get a better idea.