Resolution refers to the number of pixels in the vertical and horizontal direction--for example, my 14 inch diagonal notebook screen is SXGA+ at 1400 by 1050. A lower resolution XGA screen (1024 by 768) of the same physical size (14 inch) will logically have larger pixels, and thus your father would find them easier to read.
I believe you are thinking in terms of CRT dot pitch, where smaller generally does mean a crisper image. But I have forgotten the details, and refer you to a CRT expert on the forum.
In any event, you are looking for an LCD, and lower resolutions are easier to see.
Edit: Note that I only mentioned a 14 inch screen because that is what I own. Clearly, the larger the screen, the easier for your father to read.