Increase animal size

[HamsterKun]

The ranger in my party asked me the other day about how I’d make an oversized animal companion (more for rule of cool than anything; he kinda has the mindset). How would you increase the size of an animal? Say, upping a dire wolf from Large to Huge?

[Wizard_Lizard]

Increase HP, and movement speed?

[Spectrulus]

Going off the general guidelines in the various monster entries, bigger creatures have 10-20% more HP per CR, and lose 1-2 AC for beasts. There's often a bonus to Strength and Constitution, often only as much as +1 but occasionally +2, and a penalty to Dexterity, often -2.

[Sigreid]

I'd consider just letting it grow bigger and stronger as he advances in Beastmaster. So, keep the stats essentially the same but gradually change the size category. The idea is the bond lets the animal grow bigger and stronger.

[Desteplo]

In 3.5 they had an animal jump every three levels (could swap companion or your animal gets extra bonus hp, dmg, etc)

In a similar way you could (starting at lvl5) decrease the proficiency bonus added to a higher CR/bigger beast. With a minimum of +2

So at lvl5 you could have:
1/4CR with +3
1/2 CR with +2
Large 1/4CR with +2