Just for fun, I checked the asset size for one of our games, Swashbuckler:
Resolution Device Size
---------------------------------
480x320 iPhone 40 MB
800x480 Pandora 49 MB
1024x600 PlayBook 93 MB
960x640 iPhone Retina 100 MB
1024x768 iPad 103 MB
1280x768 BB10 108 MB
2048x1536 iPad Retina 260 MB
Each platform has assets which map pretty much 1:1 (texel to pixel), it isn't necessarily the biggest consideration in the world, but a ~50 MB download for the current Pandora could be ~260 MB if the Pandora ended up with a screen with as many pixels as the iPad Retina. Of course lower resolution assets can be used, but at that point you aren't make much use of those extra pixels anyway.
For what it is worth, I voted 16:9, 5", 720+, the aspect ratio I am not so worried about. I don't consider the Pandora to be the least bit pocketable, however this isn't based purely on size (maybe I could squeeze it into certain clothes pockets) but it is also based on fragility. I always keep my Pandora in the official carry case, I wouldn't want to put it in my pocket 'as is' which is what I do with my mobiles).
In terms of pixel density, I do find higher resolution screens look much nicer than lower resolution ones, for a Pandora (ish) sized screen, I feel 1280x800 would look nice. I would certainly be interested to see Swashbuckler running on a 1920x1080 mobile screen with 1920x1080 assets and compare that to 1280x800 mobile screen. My gut feeling is, providing the screen isn't inferior in some way (contrast, response, whatever), it will be visibly better at 1920x1080. The game will look sharper/crisper, detail on the character/terrain will be visible more clearly. I think it will be detail you'll have to look at quite closely though. So a nice extra if there, but not something you'd probably miss otherwise. However, this test case would be software specifically set up to work at all resolutions, with GUI etc. scaled based on screen size, NOT screen resolution. As _wb_ has pointed out, a heck of a lot of software scales based purely on resolution, meaning it would be pretty much unusable on a 1920x1080 screen without some help (the help perhaps being lowering the resolution in the window manager, so general desktop apps may run at 1280x800 but then full screen games can set a flag in the PND to request full 19280x1080).