The TellyMate (and it's schematically identical sister, the TellyMate Shield) can only output characters.
Each character is 8 pixels wide by 9 pixels tall.
There is a slight complication in that the last of the 8 pixels is twice as wide as the others
Technically, you could claim that means a resolution of 304 x 225 pixels, but that's really misleading, as they're not directly addressable, and are only driven via the ROM-based font (so aren't even user definable).
I have written an article about the design of the TellyMate which details how it all works.
The following diagram shows a small example of what's output:
