I have about 1000 cookbooks in my collection, and I use them frequently. Size is not important to me (except for extremely large or small books, which are awkward to use).
Here's what IS important, IMHO:
1. Large enough font to be easily read without a magnifying glass.
2. Easy to read font -- no script or other nonsense.
3. Each recipe on a single page, or at worst, on two facing pages; I hate cookbooks that require me to turn the page when my hands are covered with food.
4. Easy to follow instructions; I don't mean simple instructions, necessarily, but easy -- each step in a separate, preferably number paragraph.
5. Clear lists of ingredients, in the order they will be used, and preferably grouped by step (i.e., all the sauce ingredients in one group, the filling or whatever in another, the toppings in another); if you mean dried thyme, say dried, and if you mean fresh, say so.
6. Also helpful are weights in addition to volume measurements -- e.g., 1 cup of grated cheese (4 ounces).
7. Times can be helpful, especially for newbies -- "cream butter with sugar and beat for 5 minutes."
8. Photos can be helpful, but aren't essential.
9. I also dislike books that won't stay open in my cookbook holder.