First, I'm not sure this is the proper place, or if the question has been asked already (if so, don't know how to find it), so if I was wrong to post this here, I'm really sorry... And, just in case, sorry for my English; not my mother tongue and a bit rusty recently.

Assuming everything's fine, here's my problem.

We're a team of two, programmer and artist/script writer, and we're discussing how our art will be in our 2d platformer.

We've decided using a resolution of 1920x1080 (since, according to Wikipedia, most Steam users in May 2012 used that resolution (~20%), so that should be a good default choice so that most users have the best definition; and for smaller screens, it's just a matter of resizing dinamically).

Our characters will be tile-based and drawn in a tablet with vector graphics.

BUT our backgrounds and objects will be hand-drawn to get some nice visual effects.

So, our question is: what should be the PPI for our pictures? In Photoshop, apart from the size of the image, it's necessary to choose the PPI (pixels per inch). If we use a high PPI, our images don't lose much quality when making them small (i.e. playing the game in a lower resolution on smaller monitors), but that makes the file much heavier. So, if our standard resolution is 1920x1080 and we want our hand-drawn images to display nicely in lower resolutions, what PPI do you recommend us?

And one last thing. We've decided to use tiles of 64x64. Is it too much? Not necessary? Just plain stupid? Sorry, we couldn't find anything on the topic (not compelling reasons for one or another option, at least) and we'd appreciate some advice.

So, what about those bold things asked?

Many, many, maaaaaaaaaaaany thanks for your help :)

64 pixels will result in almost 17 rows of your tiled screen. Seems like they would be a bit small.

In this post, you can see my screen has about ten tiles vertically.

http://flatank.blogspot.com/2013/06/super-metroid.html

My current project has like 6:

http://flatank.blogspot.com/2014/02/yet-another-new-project.html

Best thing is to build it and scale it dynamically like you said- I calculate the scale based on the detected screen resolution. This bit of code show how the scale controls the size and mx,my tracks the background against foreground. A parallax effect can be achieved with an overlay which moves proportionally with background but slower.

Rect.Initialize((c*128-mx)*scale, (l*100-my+10)*scale, (c*128-mx+128)*scale, (l*100-my+110)*scale)
Canvas1.DrawBitmapRotated(greenbutton,Null,Rect,0)

PPI is irrelevant as it will differ based actual real world screen size.