framework/data/Pagination.php
public function getPageSize()
    {
        if ($this->_pageSize === null) {
            if (empty($this->pageSizeLimit)) {
                $pageSize = $this->defaultPageSize;
                $this->setPageSize($pageSize);
            } else {
                $pageSize = (int) $this->getQueryParam($this->pageSizeParam, $this->defaultPageSize);
                $this->setPageSize($pageSize, true);
            }
        }

        return $this->_pageSize;
    }
public function getPageSize()
    {
        if ($this->_pageSize === null) {
            if (empty($this->pageSizeLimit)) {
                $pageSize = $this->defaultPageSize;
                $this->setPageSize($pageSize);
            } else {
                $pageSize = (int) $this->getQueryParam($this->pageSizeParam, $this->defaultPageSize);
                $this->setPageSize($pageSize, true);
            }
        }

        return $this->_pageSize;
    }
Returns the number of items per page.
By default, this method will try to determine the page size by [[pageSizeParam]] in [[params]].
If the page size cannot be determined this way, [[defaultPageSize]] will be returned.
@return int the number of items per page. If it is less than 1, it means the page size is infinite,
and thus a single page contains all items.
@see pageSizeLimit