set ppi or pixel density or physical screen size

How do I set the pixel density for an e-document I'm working on?

I'm designing for a specific device, with a pixel density of 1200 * 1900px at 94mm * 151mm. I've set up a document (for Mobile) at the correct PIXEL size but when I set type at, say, 24pt it's miniscule. When I draw a rectangle at 20mm x 40mm it's about 5mm x 8mm.

Indesign is assuming the ppi of the device I'm designing for is much much lower (or the physical size is much greater) than it really is.

Where/how can I tell it the screen is physically 94mm * 151mm?

Thanks,

K.