Thread:Sinthorion/@comment-27722880-20151017221811/@comment-27723099-20151018155638

/summon MordorSpider ~ ~ ~ {SpiderScale:10}

This command summons a spider with the size of Shelob. You can of course vary the spider type (MirkWoodSpider instead of MordorSpider) or play around with the additional SpiderType tag (not available for MordorSpider).

With additional size, the game automatically scales the other stats of the spider too (health, speed, maybe attack damage). While a small (scale:1) spider has 12 health, 0.35 speed, and 2 attack damage, the big one (scale:10) has 30 health, 0.26 speed, and 12 attack damage. I don't know the formula for this. If you want to adjust it further, you have to set the Attribute values.