Group buffer pool sizes
The two parts of a group buffer pool are data pages and directory entries. You can specify the ratio of data pages to directory entities in a group buffer pool, or you can use the Db2 default ratio.
Data pages
Data pages (sometimes called data entries) reside in the group buffer pool. The size of a data page is the same as the page size supported by the corresponding Db2 buffer pools (4 KB, 8 KB, 16 KB, or 32 KB).
If you are caching changed data only, you need enough space to cache changed data plus extra space for pages that are frequently referenced. By caching those frequently referenced pages in the group buffer pool, you can decrease the amount of time it takes for any member to refresh that page in its member buffer pool because you avoid the disk I/O.
If you choose GBPCACHE NONE or GBPCACHE SYSTEM, no user data pages are actually stored in the group buffer pool. However, with GBPCACHE SYSTEM, space map pages for LOBs are cached in the coupling facility.
Directory entries
A directory entry specifies the location and status of a page image somewhere in the data sharing group, whether the image is in the group buffer pool or in one of the member buffer pools. Only one directory entry exists for any given page, no matter how many places that page is cached.
The size of a directory entry is approximately 400 bytes, but it varies somewhat based on the size of the data pages and the CFLEVEL you are using. 
Specifying a ratio
The amount of space that is allocated for a group buffer pool is determined by the ratio of directory entries to data pages. When you originally define a structure in the CFRM policy for a group buffer pool, you specify its total size. For Db2, the ratio defaults to five directory entries per data page. Later, you can change the ratio with the ALTER GROUPBUFFERPOOL command. The new value that you define takes effect when the group buffer pool is rebuilt or reallocated.
For group buffer pools that are defined with GBPCACHE(NO), ratios are ignored because no data is actually stored in the group buffer pool.
When possible, both a formula and a general guideline are provided to help you estimate the initial sizes and ratios of your group buffer pools. (The exception is for GBPCACHE ALL group buffer pools, for which only a general guideline is provided.)
The formula is not too complex and is likely to be more accurate, assuming that you are fairly confident of the values for the variables in the formulas. Otherwise, use the general guidelines and then adjust your values from there.