The way to do was looking at the code in code/core/Mage/Catalog/Model/Resource/Category/Flat/Collection.php
at line 380 in Magento 1.7.2 on the function setPage($pageNum, $pageSize)
$collection = Mage::getModel('model')
->getCollection()
->setCurPage(2) // 2nd page
->setPageSize(10); // 10 elements per pages
I hope this will help someone.