X height
An interesting point to notice is the variation in size of different fonts. Type is measured in points, based on the total height of the characters. This means the height of the letter from its base to the top of letters such as t or b. But fonts of the same nominal size have different x-heights. The x-height is the core size of the letter, excluding ascenders (the tall shafts of letters such as b and d that rise above the height of other letters such as n, o or x), and descenders (strokes that drop below the baseline, such as in p and q). Thus, a font with a small x-height will look very different from one with a large x-height, even if their point size is the same.

This is Verdana 11 point.
And this is Bernhard Modern BT, also at 11 point.

Though both of these are the same nominal size as the Palatino used as body text in this handbook, the Verdana looks larger, the Bernhard smaller.

Type size
All these examples I have given are 11 points in size. For books, 10 point is often the size of choice, depending on the font and the nature of the content. For many documents, including legal ones, a bigger size is more useful. For people with reading problems, 10 point is definitely too small and even 12 point can be difficult. I recommend 14 point, or certainly no less than 13, unless you are using a font with a taller x-height.

On the other hand, you do not want the font to look so big that people have to move their eyes from the beginning of the word to the end. A 16 point font may be useful for people with vision difficulties, but it can create its own problems for others.

This is Verdana 16 point.

A size this large for body text would allow only a few words per line.

Readable fonts
In my experience, a sans serif font is slightly easier for readers with literacy barriers. Some of these readers will be running a finger under each word as they read. And materials for this audience tend to be short, so there is less need to maintain continuous reading. The text also needs to be as clear and unfussy as possible.

I prefer a font such as Zapf Humanist (also called Optima) when I write for these readers. It is a modified sans serif font, with no tails, but it has the thick and thin strokes of a serifed face. I find it readable and attractive, clear without being heavy.

This is Zapf Humanist, or Optima, 14 point.

I am aware, however, that many people do not agree with my choice of a sans serif font. If you have a chance, try different fonts with your readers to see which they find easier.