The thumbnails cache is exponential ...
It is not something that you set a defined size for.
As your image files increase in number, so the cache will grow to accommodate the increase in images.
*In much the same way as the Folder itself doesn't have a predefined size (you never have free space inside a folder - just a folder that accommodates whatever you place inside it.) 
Down at the lowest form of data (at machine code level) each Folder does NOT actually exist. Each file on a HDD has a set of attributes and flags - one of these flags indicates which folder the file belongs to. This system of identification is for the benefit of the User, not the computer.
Computers are capable of keeping track of vast amounts of data all co-existing within an enormous mass of files and code. The entire directory tree structure, composed of folders and files is only presented in this form for the benefit of we mere mortals, who otherwise would not be able to find anything.
Think of a Folder and a Cache as a balloon - the more air you **** into it, the bigger it gets. The folders and sub-folders and sub-sub-folders and files and the thumbnail cache in the main Parent Directory is comparable to all the molecules of air inside the balloon, all floating around freely.
If the balloon was a computer HDD, each air molecule would be tagged so that the owner of the balloon could find one molecule from within the millions surrounding it.
Edited for *
I would like to increase the thumbnails cache for a folder in win7
Thanks for the answer.
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