special enlarged issue; bumper issue
Used for magazines, newspapers, or other periodicals that are published with extra content or larger size than usual.
This month's fashion magazine is a special enlarged issue, and the free gift is also luxurious.
Year-end newspapers often become special enlarged issues.
別冊 is a separate supplementary volume, often a standalone booklet, whereas 特大号 is still the main issue but larger.
Compound of 特大 (とくだい, 'extra large') + 号 (ごう, 'issue/number').