Yup, Maya doesn't have a "dpi" setting, however,
what I do if I need to do something larger than 4K plates is:

I go into Photoshop, create a new file at the resolution,
and size that I need.
Then I go to IMAGE > IMAGE SIZE.

And I note the pixel dimensions.

Those are then the pixel dimensions I use in Maya render global settings

Then all you gotta do is:

-render out your image in Maya.
-open it up in Photoshop.
-Go to IMAGE > IMAGE SIZE
-then change the resolution box to reflect my needed resolution.
(the res from my test image.)

-then your image will appear to resize itself to your chosen dpi dimensions.


EXAMPLE:
Say you want a 300 dpi image at a printable size of 8.5 x 11 inches, you would render an image in Maya at 2550 px X 3300 px.

Then, open it in Photoshop, go to IMAGE > IMAGE SIZE.

Change the resolution to 300, THEN input your rendered pixel dimensions in the pixel dimensions fields, and viola!

Good luck!