Hi Kutay -- This is a known issue. You've got too many customers -- a good problem to have. So, to get a successful response, you'll need to filter the results. I might suggest doing it by date. Also, you might be interested to learn that there is an undocumented API method, customers/list that works just like customers/get except it returns only current information (no invoice/transaction history). It's much more efficient than customers/get and may be a better choice for your purposes. The customers/list method suffers from the same size limitations, however, so you still might have to do your own limiting.

Long term, we will probably introduce an automatic "paging" of the response which will require multiple requests that include a pageNumber argument or the like.